A Republican-controlled Congress opens Tuesday with the most sweeping conservative agenda in decades, providing Donald Trump ample room to gut the Affordable Care Act, slash corporate tax rates and undo Obama-era environmental regulations.
The House is almost certain to reelect Speaker Paul D. Ryan (R-Wis.) as its first order of business, dispensing with the messy political infighting that has hobbled Republicans in the past.
And the Senate will swiftly begin vetting the president-elect's most controversial Cabinet picks, ready to confirm some when Trump is inaugurated as president on Jan. 20.
Yet Republicans remain at odds on some high-profile issues — such as how aggressively to investigate Russian hacking in the 2016 election — and how to fulfill other big-ticket promises, such as replacing Obamacare.
Despite firm Republican control of both the White House and Congress, the internal disputes have left them without a clear plan yet for Trump's first 100 days, or an endgame for the two years of the 115th Congress.
Trump's often shifting views on major issues will test relations with GOP's leaders on Capitol Hill, and his willingness to skirt ideological rigidity gives incoming Senate Democratic leader Charles Schumer of New York and House Minority Leader Nancy Pelosi of San Francisco an opening to influence and shape the president's evolving agenda.
President Obama will visit Capitol Hill on Wednesday to meet with Democrats bracing for their new role, not just as the minority party, but as the main roadblock preventing Trump from dismantling the healthcare law and other parts of the Obama agenda.
Republicans will also assemble behind closed doors. Vice President-elect Mike Pence, who was a popular conservative congressman before he was elected governor of Indiana, is likely to serve as a crucial link between the Trump administration and its allies in Congress.
Given Trump's inexperience in government, Pence is expected to play an enhanced role, perhaps like the one former Vice President Richard Cheney held under President George W. Bush.
At a minimum, Pence could provide a vital conduit between the untested new president and his more ideological party members in Congress, especially as Ryan's own relationship with Trump has been strained.
Ryan flip-flopped over Trump — first withholding his endorsement, then ultimately campaigning for him — but the speaker insists he and the president-elect have let bygones pass as they talk almost daily on their plans for fulfilling Republican promises to voters.
"Very soon after the race, Donald and I said: 'Look, this is fantastic. We have so much to do. Let's forget about, you know, any differences in the past and let's get working on this agenda,'" Ryan said recently on Fox News. "And that's exactly what we've been doing from -- that day on."
Once Trump takes office, Republicans will face enormous pressure to score some legislative wins after six years of trying to block most of Obama's initiatives.
Lawmakers will vote this week on low-hanging fruit -- a popular GOP measure to rein in the executive branch by requiring congressional approval for new federal regulations with an economic impact of more than $100 million.
The measure, which passed the Republican House three times since 2011, is a GOP priority. Its supporters say it would have prevented nearly all the climate and employment rule changes of the Obama era.
Republicans are also expected to punish Democrats for last year's gun control sit-in, led by civil rights icon Rep. John Lewis (D-Ga.), by imposing new rules that would slap up to $2,500 fines on lawmakers who film such floor protests from smartphones or other devices.
Whether that would pass judicial review is less clear. Opponents say the proposed ban is clearly unconstitutional.
But the GOP's top promise — to end Obamacare — remains a tough haul.
Votes are expected in coming days on legislation to begin repealing the Affordable Care Act. But these first steps will be largely symbolic while lawmakers debate the details of dismantling the healthcare law.
With 20 million Americans now benefiting from Obamacare, the GOP's gutting of it comes with an asterisk.
Republicans are also likely to postpone fully dismantling the healthcare law until they can sort out their own ideas for a yet-to-be-determined alternative.
That could push a full Obamacare repeal and replace until 2018 or 2019, after the midterm elections.
"Repeal and delay, it doesn't even have alliteration," Pelosi scoffed on a conference call Monday with reporters. "It's an admission that it's a lot for them to lose politically."
Similarly, Republicans are still working out the details of tax reform beyond the lower rates proposed in the House GOP's "Better Way" blueprint agenda for the new year.
Ryan will almost certainly reemerge as speaker in Tuesday's floor vote. But his leadership remains constrained by the same internal party divisions that hobbled his predecessor, John A. Boehner (R-Ohio), and that have prevented Republicans from making gains on a cohesive agenda.
Those divisions surfaced late Monday when Republicans split during a closed-door session over a proposal to gut a congressional ethics office at a time when Trump has promised to come to Washington and "drain the swamp." Approval of the proposal signaled a rocky start to the new session.
Democrats, despite being in the minority in both the House and Senate, will be more than bit players in the new Washington environment.
Senate Democrats are planning a robust grilling of Trump's Cabinet choices, particularly Rep. Tom Price (R-Ga.), the conservative orthopedic doctor who has been tapped to helm the Health and Human Services Department and the Obamacare unraveling.
Democrats are also lining up against Sen. Jeff Sessions (R-Ala.) as attorney general, and have raised questions about ExxonMobil chief executive Rex Tillerson, who has substantial ties with Russia, as secretary of State.
Senate Majority Leader Mitch McConnell (R-Ky.) intends to have several of Trump's Cabinet picks – likely the national security team -- cleared for quick confirmation after Trump takes office.
But McConnell faces his own difficulty as several top Republicans, including Sens. John McCain of Arizona and Lindsey Graham of South Carolina, are pushing for an aggressive investigation in Congress of Russian cyber-attacks during the presidential race. McCain plans to hold his first hearing on the issue Thursday.
McConnell has resisted calls by a bipartisan group, led by McCain and Schumer, for creation of a separate special committee – or an independent panel like the bipartisan commission that investigated the terrorist attacks after Sept. 11, 2001.
Many Republicans are optimistic for the days ahead, ready to "hit the ground running," Ryan has said.
Congress adjusted its calendar to work more, with several five-day work weeks in the House as it races to deliver on election promises. Normally the House is in session only four days a week.
9:05 p.m.: The story was updated with the news that Republicans voted during a closed-door session to gut a congressional ethics office.