The Republican field in the Arizona U.S. Senate race is expected to add two prominent names this week: Former Sheriff Joe Arpaio and Rep. Martha McSally. Both present a challenge to the current front-runner, former State Senator Kelli Ward. The incumbent Republican Senator, Jeff Flake, is retiring.

The Washington Examiner reports that "Arpaio, a close ally of President Trump and former sheriff known for his provocative approach to combating illegal immigration, is running for Senate in Arizona." Meanwhile, Politico's Kevin Robillard tweets that Arizona Republican Rep. Martha McSally has "special announcements" set for Friday. It is likely that she will enter the race at that time.

McSally has events scheduled beginning in Tucson then moving on to Phoenix and Prescott. Her 2nd district covers the Southeastern corner of Arizona, including much of Tucson. It was reported in early November that McSally was planning to join the race, so her expected announcement will not be a surprise.

The new entrants set up a three-way battle for the Republican nomination. McSally, in her 2nd term in Congress, is the preferred candidate of the establishment. The conservative Ward has received support from President Trump and an endorsement from Steve Bannon, although she's distanced herself from Bannon in recent days. Arpaio is likely to appeal to many of the voters drawn to Ward. This could actually ease McSally's path to the nomination.

Whomever emerges as the nominee will likely face Democratic Rep. Kyrsten Sinema in the general election. That contest is seen as a toss-up at this time. 

McSally's district is also seen as a toss-up. While she won reelection by 14% last November, Hillary Clinton won the district by 5%. It is one of just 23 districts nationwide that elected a Republican to Congress and voted for Clinton over Trump.

Fox News reports that "House Foreign Affairs Committee Chairman Ed Royce on Monday became the latest influential Republican lawmaker to announce plans to retire from Congress at the end of his term." Royce is in his 13th term, and represents California's 39th district which sits just east of Los Angeles. He is the first of California's 53 House members to announce they are not seeking reelection this year.

While Royce won reelection by nearly 15% in 2016, the district is one of just 23 nationwide that elected a Republican to Congress and voted for Hillary Clinton over Donald Trump. Clinton won here by 8.6%. With the incumbent retiring, Sabato's Crystal Ball has moved this race to toss-up from Leans Republican.

Royce brings to 44 the number of House members retiring or seeking another office; that's over 10% of the body's 435 seats.

Florida Republican Rep. Ron DeSantis has joined the state's 2018 race for governor. Incumbent Republican governor Rick Scott cannot run again due to term limits. DeSantis, who is in his 3rd term in the House, was recently endorsed by President Trump for the position.

Congressman Ron DeSantis is a brilliant young leader, Yale and then Harvard Law, who would make a GREAT Governor of Florida. He loves our Country and is a true FIGHTER!

— Donald J. Trump (@realDonaldTrump) December 22, 2017

DeSantis represents the 6th district, which runs from south of Jacksonville through Daytona Beach to north of Orlando. Both DeSantis and Donald Trump won here by 17 points in 2016, and the district is seen as safe Republican.

The gubernatorial race is one of 36 to be held in 2018. It is considered a toss-up at this point.

43 House members have announced they aren't running in 2018. One of them, Ohio's Pat Tiberi, will be leaving Congress on January 15th. Gov. John Kasich has set August 7th as the date for a special election, which will be preceded by a primary on May 8th.

Mississippi Republican Gregg Harper announced Thursday that he would not seek reelection in 2018. Harper, who is in his 5th term, represents the state's 3rd district, a safe Republican area that Donald Trump won by nearly 25 points in 2016. The district crosses the state, from the border with Louisiana to that with Alabama. It includes part of the Jackson area, as well as Meridian.

Related: Full list of House Retirements

Separately, Ohio Republican Pat Tiberi, who announced this past October that he would leave Congress by the end of January, has set January 15th as his departure date. Gov. John Kasich, who represented the 12th district prior to Tiberi (both of them served nine terms) will set the date for a special election. Donald Trump won this district by about 11% in 2016 and it likely will remain in Republican hands.

Pennsylvania Republican Rep. Bill Shuster announced Tuesday he will not seek reelection in 2018. Shuster is in his 9th term, and chairs the House Transportation and Infrastructure Committee.

Shuster represents Pennsylvania's 9th district, which covers a large part of the south-central part of the state. The seat is safely Republican. Donald Trump won the district by over 42 points in 2016, his largest margin anywhere in the state. 

Shuster becomes the third Pennsylvania U.S. House member - all Republicans - to pass on the 2018 race. Rep. Charlie Dent (PA-15) is also retiring, while Rep. Lou Barletta (PA-11) is running for U.S. Senate. 

This is the first House retirement announcement of 2018. There are now 41 members on the list of departures, including 27 Republicans and 14 Democrats. One of the Republicans, Ohio's Pat Tiberi, is expected to leave Congress this month.

Utah Republican Senator Orrin Hatch announced he will retire at the end of this year. The departure opens the door to a likely run by Mitt Romney, the 2012 Republican presidential nominee and former Massachusetts governor. Romney reportedly indicated to associates last year that he would enter the race if Hatch chose not to run.

Now in his 7th term, Hatch is the longest-serving Republican in the Senate. He joined in early 1977 after winning in 1976. Only Vermont's Patrick Leahy, who entered the Senate in 1975, has served longer. Leahy was elected to an 8th term in 2016.

Romney would be the prohibitive favorite to win should he choose to run.

Two new Senators will be seated Wednesday as the 2nd session of the 115th Congress convenes. Alabama Democrat Doug Jones will replace Republican Sen. Luther Strange, while Democratic Minnesota Lt. Gov. Tina Smith will fill the seat being vacated by fellow Democrat Sen. Al Franken.

Jones defeated Republican Roy Moore in a special election on December 12th. He is the first Democrat to win a Senate election in Alabama since 1992, when Richard Shelby was reelected to a 2nd term. Shelby would become a Republican two years later. Now in his 6th term, Shelby remains the state's senior Senator. Jones will serve the remainder of the term. The seat is up again in 2020.

Smith was appointed by Minnesota Gov. Mark Dayton to replace Franken. She is expected to face the voters in a special election this November. The winner there will complete the remainder of Franken's term. This seat is also up again in 2020.

The Democratic gain in Alabama will bring the Senate to 51-49 Republican. Democrats will need to pick up two seats to gain control in 2018.

Minnesota Sen. Al Franken has set January 2, 2018 as his resignation date. His replacement, Lt. Gov. Tina Smith, will be sworn in the following day. She was appointed to the position by Minnesota Gov. Mark Dayton after Franken announced his intention to resign earlier this month. 

When Smith is seated, Minnesota will become just the 6th state to ever have two female Senators serving simultaneously. The other five include California, New Hampshire, Washington, Kansas and Maine. The first three states in that list currently have two women in the Senate.

Smith's appointment is temporary. Minnesota will hold a special election coinciding with the midterm elections next November. The winner will serve the final two years of Franken's term. That seat's next regularly scheduled election is in 2020.

Minnesota's other Senator, Amy Klobuchar, is also up for reelection in 2018. According to an analysis by Sabato's Crystal Ball, this will be the 55th time since 1913 that a state has had a 'double-barrel' Senate election. The analysis shows that in the vast majority of cases, the same party wins both elections. In fact, this has been the case 100% of the time since 1966. 

Use our Senate Interactive Map, now updated for Alabama and Minnesota, to create and share your own 2018 Senate election forecast.

The Census Bureau released its annual national and state population estimates on Wednesday, noting that Idaho was the nation's fastest growing state for the year ending July 1, 2017. Election Data Services has extrapolated population growth from 2010-17 through to 2020, when the next Census will take place.

Among other things, the Census will determine how the 435 congressional districts will be reallocated next decade. This, in turn will determine changes to the electoral map. Each state receives an allocation of electoral votes equal to the number of congressional districts it has plus two for its Senators. 

At this point, Texas and Florida look to be the big winners, gaining 3 and 2 electoral votes, respectively. Arizona, Colorado, North Carolina and Oregon will gain one each. As the number of electoral votes is fixed, these 9 increased votes must come from somewhere else. The current projection is that Alabama, Illinois, Michigan, Minnesota, New York, Ohio, Pennsylvania, Rhode Island and West Virginia are on track to lose one each. This projection is virtually unchanged from late 2016. The only difference is that Illinois, as opposed to Texas, would be allocated the final congressional district.

Of course, much can change between now and 2020. For example, hurricane Maria devastated Puerto Rico this summer - after the July 1 estimate date - leading to an outward migration of those residents to Florida. It is unknown how many of those will return to the island when life gets back to normal there.

We've created interactive maps of the last five presidential elections using the projected 2024 map. There's little change from 2016, where Donald Trump would win two more electoral votes with the 2024 map. However, looking back to 2000, George Bush would have won 19 more electoral votes, giving him a wider 290-248 victory over Al Gore. That said, Florida would still have made the difference. The Sunshine State is projected to have 31 electoral votes in 2024, vs. 25 in 2000.

The U.S. House of Representatives passed the tax reform bill Tuesday. The measure will now head to the U.S. Senate. If approved, it will then forwarded to President Trump for his signature. 

227 Republicans supported the measure, while 12 opposed. 191 Democrats voted against the measure, while 2 did not vote. Of the 12 Republicans voting no, 11 are from California, New Jersey, or New York, all states with areas of expensive real estate, as well as a high income and/or property tax burden. The bill places limits on the amount of these taxes that can be deducted, as well a reduced mortgage interest deduction.

Curious how your representative voted? You can check here. That page also shows how each representative in a competitive 2018 district voted. Click or tap a state to see how each member of that state's delegation voted.

Related: Will your taxes go up or down? Use this interactive calculator to find out.