Republicans took control of both chambers of the US Congress on Tuesday, setting the stage for fierce partisan battles that could consume the final two years of Barack Obama's presidency.
In the Republicans' sights: Obama's signature health care law, key environmental and business regulations, and the president's recent executive actions on immigration sparing millions from deportation.
The party is emboldened after pummeling the Democrats in the November elections, taking the Senate and expanding their majority in the House of Representatives. They are now in charge of both branches of Congress for the first time since Obama took office.
The Republicans planned to move swiftly toward a showdown with the White House over the long-stalled Keystone XL oil pipeline, summoning unity despite a tea party-backed effort to unseat House Speaker John Boehner. "We will get to work right away," pledged House Majority Leader Kevin McCarthy. "And with a new Republican-led Senate, we anticipate many of these bipartisan solutions to finally reach the president's desk."
At the White House, Obama planned to meet with the new congressional leadership next week as both sides positioned themselves for two years of clashes and, perhaps, occasional cooperation that will help shape the outcomes of the 2016 presidential and congressional elections.
The president has sent a message that he's still relevant and infuriated Republicans with a series of high-profile presidential moves, including steps toward normalizing relations with Cuba. And no matter what the Republican Congress passes, Obama retains the power to veto legislation, an action he's taken only twice in six years.
As mandated by the Constitution, Congress convened at noon (1700 GMT) Tuesday.
At the top of the Republican agenda is a veto showdown with Obama over the oil pipeline from Canada to Texas.
The legislation, opposed by environmental groups and many rank-and-file Democrats, passed the House but died in the Democratic-led Senate late last year. Republican leaders now intend to push the bill through the House late this week, and appear to be close to having enough votes to clear it through the Senate as well.
While Obama has not said if he will reject the measure, White House Press Secretary Josh Earnest outlined a series of concerns before adding, "I'm not prepared at this point to issue a veto threat related to that specific piece of legislation."
Despite the looming clashes, Republican leaders and the White House have indicated there is potential for cooperation on issues like trade, tax reform and infrastructure spending. The challenge for leaders of both parties will be facing down pressure from their more ideological members.
Obama might face pressure from his party's liberal wing if he tries to work out a deal with congressional Republicans on the Trans-Pacific Partnership, a trade deal that 12 nations are trying to negotiate.
Senate liberals, led by Democratic Sen. Elizabeth Warren and independent Sen. Bernie Sanders, have declared opposition to the trans-Pacific trade deal, contending it would benefit global corporations at the expense of American workers.
Boehner, the leader of the House, has seen his efforts to forge compromises thwarted in recent years by lawmakers aligned with the conservative tea party movement. But his hand is considerably stronger this year as a result of the Republican electoral triumph. The party will hold its biggest House majority in nearly 70 years.
First, Boehner must be re-elected as speaker. His prospects of getting a new term appeared secure, despite challenges from at least 10 Republicans, including tea party-backed members.
Many Republican lawmakers dismissed the challenge as a needless distraction at a moment when the party should be celebrating its new majorities and showing voters it can lead.
Another distraction for the party has been revelations that the House's No. 3 Republican, Majority Whip Steve Scalise, spoke to a white supremacist group 12 years ago.
The White House waded into the furor Monday, with Earnest saying it's up to Republicans to decide whether Scalise will retain his job and that who the party has in leadership "says a lot about who they are."
In the Senate, Kentucky Sen. Mitch McConnell's ascension Tuesday to the post of Senate majority leader was automatic following his approval by rank-and-file Republicans late last year. McConnell, who had been the Senate minority leader, faced criticism from tea party supporters over his role in brokering a bipartisan spending deal that ended a government shutdown in 2013. But he survived a tea party primary challenge last year and went on to handily win re-election to his Kentucky seat in the general election.