The Note: Could Romney's entry in Utah Senate race serve as Trump foil? Romney’s expected candidacy in the Utah Senate race sets up a defining battle.

 -- The TAKE with Rick Klein

Two new U.S. senators get sworn in today from Alabama and Minnesota, providing some punctuation to close out a few of the bigger storylines of 2017.

But one of the biggest stories of 2018 is only now just developing. Mitt Romney’s expected candidacy in the now-open Utah Senate race – former aides and advisers do expect him to take the plunge – sets up a defining battle for the Republican Party in the midterm election year.

It’s not just that the former Massachusetts governor and two-time presidential contender would come to the race with virtually unmatched celebrity and credibility. It’s not just that Romney has found fit to speak his mind about President Donald Trump, during the long campaign and during key points of his presidency.

It’s that Romney is the antithesis of Trump - in style, behavior, and political instincts. Romney and his establishment bona fides represented what Trump sought to destroy in his rise to power; to the minds of many people who call themselves Romney supporters, Trump embodies the worst of the modern GOP’s instincts.

Yes, Trump endorsed Romney in 2012, and yes, Romney came close to serving as Trump’s secretary of state. But say Romney’s name at a Trump rally – or dial Steve Bannon up on the subject, as a few Senate challengers might – and the chasm between the two men and what they represent becomes evident.

The Utah Senate race is unlikely to determine control of the Senate. But it just might go a long way toward determining control of the Republican Party in the era of Trump.

The RUNDOWN with MaryAlice Parks

Welfare reform, infrastructure, immigration and health care.

Those are the White House’s stated, top four legislative priorities for the New Year, and, interestingly, each one of them holds the potential for a bipartisan deal.

All the ingredients are there. Stated desire to work across the aisle? Check.

Middle-of-the road proposals already vetted and available? Check.

Urgency? Check.

Necessity? Yea, probably.

Democrats have shown an ability to stick together in the last year, whereas Republicans, until the latest tax vote, struggled to rein in everyone in their party.

Now, with midterms on the horizon and the president’s approval ratings what they are, getting all GOPers to vote together is likely to get even harder. Add in the fact that the Republicans’ Senate majority is about to shrink this week and the likelihood that the White House will need Democratic votes to get anything done – skyrockets.

Democrats may hold enormous negotiating power in the coming weeks, if Republicans let them. How they chose to wield that power – which red lines they set, which policies they prioritize – will be key.

The TIP with Arlette Saenz and Meridith McGraw

Julián Castro, the former Obama administration Secretary of Housing and Urban Development who’s often mentioned as a potential 2020 Democratic presidential candidate, launched his Opportunity First PAC Tuesday focusing on supporting young progressive candidates for federal, state and local offices.

The PAC’s other goals are to flip the House in 2018 and take control of key state legislatures before redistricting happens based on the 2020 U.S. Census.

The Opportunity First PAC has already thrown its support behind former NFL player Colin Allred who’s one of several Democratic candidates vying for Texas Republican Pete Sessions’ seat in Congress and two successful candidates for Virginia state legislature, Kathy Tran and Lee Carter.

Castro plans to travel and campaign for the candidates his PAC supports.

WHAT YOU NEED TO KNOW TODAY

In his latest war of words with North Korean leader Kim Jong Un, President Donald Trump took to Twitter to attack Kim’s “depleted and food starved regime” and taunt the rogue leader with his “bigger and more powerful” nuclear button that “works.”

President Trump will have lunch with Vice President Mike Pence, Secretary of State Rex Tillerson and Defense Secretary James Mattis.

Congress gavels in the second session of the 115th Congress. The Senate gets down to business today while the House returns Monday for legislative business.

Office of Management and Budget director Mick Mulvaney and legislative affairs director Marc Short will meet with the big four congressional leaders at Speaker Paul Ryan’s office to discuss the 2018 congressional agenda.

Tina Smith, Minnesota’s lieutenant governor, will be sworn in today as a U.S. senator replacing Al Franken. This brings the total number of women in the Senate to a record-breaking 22, according to Rutgers’ Center for American Women and Politics.

After defeating Republican candidate Roy Moore, Doug Jones will also be sworn in today as Alabama’s newest Democratic senator. According to AL.com, former Vice President Joe Biden will escort Jones for the ceremony, breaking from tradition.

QUOTE OF THE DAY

"Every good fighter knows when to hang up the gloves.” — Sen. Orrin Hatch, R-Utah, in a video posted on his Twitter announcing he will not seek re-election.

NEED TO READ

Donald Trump to Kim Jong Un: My 'nuclear button' is 'much bigger and 'more powerful' than yours. Size matters. So says President Donald Trump, who ignited a new war of the words Tuesday with North Korean leader Kim Jong Un over the size of his purported "nuclear button." (ABC News) http://abcn.ws/2lGfXNw

White House to meet with Hill leadership on 2018 agenda. President Trump is dispatching his top aides to the Hill Wednesday for talks with leadership about his 2018 legislative agenda. (Alexander Mallin and Benjamin Siegel) http://abcn.ws/2CFQ9bx

Utah Sen. Orrin Hatch to retire at end of term after 40 years in Senate. Sen. Orrin Hatch, the most senior Republican in the United States Senate, and the body's president pro tempore, announced Tuesday that he will not seek re-election at the end of his term this year, capping a 40-year tenure representing the state of Utah. (Mariam Khan and Adam Kelsey) http://abcn.ws/2CJ3j7S

Romney changes his Twitter location from Mass. to Utah amidst Senate run speculation. The change in location, which may appear seemingly benign, comes as speculation swirls that Romney may be interested in filling Republican Utah Sen. Orrin Hatch's Senate seat. (David Caplan) http://abcn.ws/2DVZ2NE

Trump goes after former Clinton aide Huma Abedin in tweet. President Donald Trump in a tweet Tuesday morning appeared to suggest that a top aide to Hillary Clinton should be punished and possibly jailed for her handling of certain emails while at the State Department. (Meghan Keneally) http://abcn.ws/2A7Skll

What's on Congress' already-lengthy to-do list. Republican members of Congress likely spent the holidays toasting their tax bill victory, but they face a long to-do list when they return to Washington today. Here’s a look at what we already know is on Congress’ 2018 to-do and wish lists. (Ali Rogin) http://abcn.ws/2CraljZ

Doug Jones hires Senate Democrats' only African-American chief of staff. Sen.-elect Doug Jones, the Democrat from Alabama who beat Republican Roy Moore in last month's special Senate election, has tapped former Department of Transportation staffer Dana Gresham as his chief of staff, making him the only African-American chief of staff for a Senate Democrat. (David Caplan) http://abcn.ws/2lIom38

Al Franken marks last day as U.S. senator after sexual misconduct accusations. Minnesota Democrat Al Franken served his last day as a U.S. senator Tuesday. (Mariam Khan) http://abcn.ws/2CI4KmT

American service member killed in eastern Afghanistan. An American service member was killed in combat in eastern Afghanistan on Monday and four others were wounded in an area known for ISIS activity. (Luis Martinez) http://abcn.ws/2CdQDVc

Former President Obama reveals his favorite books and songs of 2017. Former President Barack Obama hasn't let go of some of the traditions he adopted in the White House. One of them is sharing his favorite books and songs from the previous year, 2017 highlights he released this time on New Year's Eve. (Meghan Keneally) http://abcn.ws/2lJDsUX

In reversal, FEMA says houses of worship now eligible for disaster relief. Following a lawsuit filed by three churches damaged in Hurricane Harvey last year, FEMA has reversed a policy that prevented houses of worship from accessing disaster relief funds, according to a guidance document published by the agency Tuesday. (Erin Dooley) http://abcn.ws/2CvMVK5

In an op-ed published in the New York Times, the co-founders of Fusion GPS – the research firm that commissioned the dossier on Donald Trump’s ties to Russia – pushed back on reports made by some Republican legislators that the dossier ignited the FBI’s investigation into Russian meddling. http://nyti.ms/2ChGhUx

The Washington Examiner reports that the attorney who Roy Moore's wife touted employing in response to accusations of anti-Semitism is actually a longtime friend and supporter of Doug Jones. http://washex.am/2CFOUZR

The Note is a daily ABC News feature that highlights the key political moments of the day ahead. Please check back tomorrow for the latest.