John Boehner is about to face the reality of the power he now holds.

The Ohio Republican, who will be elevated from Minority Leader to Speaker today, is up against an enormous challenge: He must appease a flock of conservative and ambitious GOP freshmen while putting in place a strategy for his party to expand its Congressional majority and win the White House in 2012.

The 112th Congress already is shaping up to be highly adversarial, with partisan showdowns looming over health care reform, federal spending and the debt, and a proposed Republican House rules package that would allow, among other things, Budget Chairman Paul Ryan (R-Wis.) to craft a budget without Democratic input.

Democrats are locked in an internal debate over what kind of minority party to be after four years of wielding the Speaker’s gavel and controlling the House agenda.

Minority Leader Nancy Pelosi — the first female Speaker — is far weaker than she once was: The California Democrat is expected to face public defections on the House floor today from unhappy moderate Democrats who still blame her for the loss of their majority on Nov. 2 and believe she should have stepped aside to make way for new leadership. Pelosi won her bid in November to serve as leader in the new Congress, although 43 Members chose Rep. Heath Shuler (N.C.) instead.

The official transfer of House power is set to occur this afternoon after Dean of the House John Dingell (D-Mich.) administers the oath of office to Boehner, who will win the support of a majority of House Members to serve as Speaker. Republicans are expected to unanimously support Boehner, while most Democrats will back Pelosi. However, Shuler intends to challenge Pelosi again, voting for himself for the top House job. A number of other like-minded Democrats have indicated they will follow Shuler’s lead.

In his speech today, Boehner will focus on his plans to reform House operations and the “need for the House to listen to the American people, and do their will,” while also reminding Members that they cannot afford to avoid “the tough choices that need to be made about our nation’s future,” according to one aide to the incoming Speaker. Boehner is also expected to use the address to once again tell his life story — stressing his blue-collar upbringing as the son of a bar owner in western Ohio.

Boehner has been preparing for his new job for weeks. But one senior Democratic aide said the Speakership is far different from the role of Minority Leader.

“He’s no longer the head of a minority that’s trying to rebuild their base and make gains ahead of the next election,” the Democratic aide said. “He’s the head of the whole Congress, essentially.”

Boehner, 61, will have a difficult balancing act. His Conference has many new Members inspired by the tea party movement, and many of them expect him to push a sweeping, anti-spending, small-government agenda with an emphasis on the Constitution. At the same time, Boehner will face pressure from Democrats trying to paint him as an extremist who is out of touch with mainstream America, and perhaps more importantly, independent voters.

One of the biggest tests of his new leadership will be during the debate over raising the debt ceiling, which many new Republican Members have vowed to oppose. An impasse on the issue could force a government shutdown.

But Republican aides said they aren’t worried about an intraparty divide.

“On the big issues facing this Congress — repealing and replacing ObamaCare, cutting spending and helping the economy create jobs — there is zero daylight between our Members,” one Republican leadership aide said.

A former House Republican leadership aide also downplayed the prospect of a schism between Boehner and his right flank, saying criticism of leadership “is going to be natural part of who the tea partyers are” in Congress.

“There will be times when the House Republican leadership and the tea partyers will be naturally at odds, and that’s not necessarily a bad thing,” the former aide said.

Boehner is already giving a few nods to the tea party: On Thursday, the Constitution will be read aloud on the House floor, and Members will vote on a resolution that would reduce the operating budgets of House committees, leadership offices and individual Member offices by 5 percent. Republicans said the move would save taxpayers an estimated $35 million.

Majority Leader Eric Cantor (Va.) on Tuesday labeled the new GOP majority a “cut and grow” party, citing plans to bring up spending cuts and deficit-reduction measures each week Members are in session.

But also on the agenda is an assault on what Republicans view as “job-killing” Obama administration regulations.

Incoming Energy and Commerce Chairman Fred Upton (R-Mich.) plans to investigate the Environmental Protection Agency’s recent announcement to limit greenhouse gas emissions from power plants and other industrial sites, while Oversight and Government Reform Chairman Darrell Issa (R-Calif.) has said he wants to launch six major investigations during the first three months of the year, including inquiries into Fannie Mae and Freddie Mac’s role in the foreclosure crisis, recalls by the Food and Drug Administration, and the release of classified government cables by WikiLeaks.

Republicans appear unified now, but GOP aides and K Streeters acknowledged that the new majority will have a tougher time sticking together going forward, particularly once they begin the discussion over how to replace President Barack Obama’s health care law. House Republicans plan to vote next week on repealing it; Republicans campaigned on promises to “repeal and replace” the measure.

Democrats are also working on a strategy for the 112th Congress: “If Republicans believe that they can implement a very much out-of-the-mainstream, right-wing agenda, they will quickly see that whatever initial support they have will erode,” the senior Democratic aide said. “This idea that it’s going to be our way or the highway is just not in keeping with what the public wants.”

Democrats huddled behind closed doors Tuesday afternoon to figure out how best to counter the GOP’s agenda, including the rules package and health care repeal. Democrats have described Republicans’ health care repeal efforts as political theater and have vowed to oppose it.

Pelosi said Tuesday that the repeal would “do severe, very serious violence to the national debt and deficit.”

One of Pelosi’s chief tasks as Minority Leader will be acting as a bulwark against Republican attempts to chip away at initiatives such as health care that she helped enact as Speaker. But Pelosi still says she has “no regrets.”

“I don’t really look back,” she said. “I look forward.”

Pelosi pledged that Republicans “will have a willing partner in solving problems for the American people,” but only when they “have positive solutions.”

Jessica Brady contributed to this report.