Optimistic Republicans assumed full control of Congress on Tuesday for the first time in eight years in a day of pomp, circumstance and raw politics beneath the Capitol Dome.

Before the new Congress was two hours old, a veto showdown with President Barack Obama was set as the White House announced he would reject legislation approving the long-stalled Keystone XL pipeline, which Republicans intend to advance.

"Hard work awaits," said the new Senate majority leader, Mitch McConnell of Kentucky. "I'm really optimistic about what we can accomplish."

House Speaker John Boehner, who faced down a tea party challenge and won re-election Tuesday by his colleagues, said the GOP will seek Obama's support for measures on jobs, energy and veterans. "It will be a sign the logjam is breaking," Boehner declared.

At the White House, Obama congratulated the pair and said he was looking forward to working with them.

"There are going to be areas where we disagree, and there will be some pitched battles, but I'm also confident that there are enormous areas of potential agreement," Obama said. "I wish them well and I think we're going to hopefully have a productive 2015."

As the first snowfall of the winter blanketed Washington, Congress convened at noon as required by the Constitution.

On the Senate floor, newcomers mixed with veterans as Vice President Joe Biden swore in senators in groups of four. McConnell ascended to majority leader, his elevation endorsed by rank-and-file Republicans last year after they won control of the chamber from the Democrats.

Across the Capitol in the House, a similar scene unfolded as familiar faces and new ones crowded the aisles and lawmakers of both parties recited the Pledge of Allegiance. But in the House, there was an element of suspense as Boehner faced a tea party-backed effort to unseat him.

It came up short, but the 25 defections was a historically high number for a sitting House speaker. It served notice that the conservative faction that has been a thorn in Boehner's side for the past two sessions of Congress will not fall quiet during the new one despite the GOP's bigger control.

Seeking unity despite the internal party dissension, the GOP moved swiftly to advance the Keystone XL pipeline, setting votes in a Senate committee and on the House floor for later this week.

After months of equivocating, the White House announced Tuesday that Obama would not sign the bill. Spokesman Josh Earnest said there is a "well-established" review process that is being run by the State Department that should not be undermined by legislation. "If this bill passes this Congress, the president wouldn't sign this bill," Earnest said.

It was a preview of things to come 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.