WASHINGTON — Firmly under Republican control for the first time in four years, the U.S. House of Representatives promises to become the key battleground for a coming assault on Obama administration policies, from health care to taxes to federal spending.

The GOP roared past the 39-seat gain needed to retake control of House in Tuesday’s election and appeared headed to a historic rout — with some projections showing an increase of 50 or more seats, threatening to surpass the 52-seat swing in 1994’s so-called “”Republican Revolution.””

The decisive Republican surge raised the possibility of the Democratic Party falling below 200 seats in the 435-seat House for the first time since 1948.

John Boehner of Ohio, expected to be the new Speaker of the House, pledged an aggressive agenda in remarks at a rally in Washington late Tuesday. “”This is not a time for celebration,”” Boehner said. “”This is a time to roll up our sleeves and get to work.””

Boehner grew emotional as he spoke, nearing tears as he said, “”I’ve spent my life chasing the American dream,”” and the ballroom crowd chanted, “”USA!”” It was a notable moment that captured how far the party had come from two years ago, when it lay at its lowest ebb.

The wave crashed down on Democratic veterans and first-termers alike, sending them to defeat in every region of the country. The GOP was close to claiming four seats in traditionally Democratic New York state, while Democratic committee chairmen such as Reps. John Spratt of South Carolina and Ike Skelton of Missouri went down.

All year, Republican candidates have run against the polices set forth by President Barack Obama and the Democratic-controlled Congress. But campaigning and governing are vastly different missions — a lesson that was imparted to congressional Democrats in this midterm election. Now, many of those victorious GOP candidates will be charged with legislating, rather than tossing barbs at the opposition.

Boehner would preside over what could be an unruly majority, filled with a bevy of neophyte representatives who have vowed to shrink the size of government, curtail federal spending and repeal the health care overhaul Obama made a centerpiece of his early agenda.

At the same time, Boehner and the GOP will be tasked with keeping the government running, an issue that could surface early in his reign if the lame-duck Congress fails to pass a sweeping funding measure in the next month.

“”He’s got to figure out how to put together a majority that can vote to fund the government,”” said Vin Weber, a former Republican congressman from Minnesota. Many of the new members “”will be allergic to spending votes.””

Given the heavy Republican gains, Boehner will have some breathing room. He’ll also potentially be able to attract moderate Democrats — that is, the ones who survived Tuesday’s election — who will be eager to demonstrate their conservative credentials before running again in 2012.

“”There’s an opportunity to put together a pretty broad bipartisan coalition,”” Weber said.

But gridlock is also a possibility — given that the Senate was projected to remain Democratic and Obama will be in the White House, ready to veto any bill that threatens his agenda. For that reason, a complete repeal of the health care legislation is unlikely, although that won’t stop the Republicans from bringing the issue to the floor.

Rep. Eric Cantor of Virginia, expected to become the new GOP majority leader, said Tuesday night that his colleagues have learned from the mistakes and excesses that led to their ouster four years ago. “”We Republicans are a different party from the GOP of 2006,”” Cantor said in Richmond, Va. “”Our years in the minority have chastened and disciplined our party.””

But some Republicans are thirsting to use their new oversight power to make life tough for Obama. “”Do I expect to have the hardest working committee on the hill? I do,”” said Rep. Darrell Issa of California, in line to take over the Oversight and Government Reform Committee.

Boehner said he will make reducing the deficit and cutting spending early priorities. Unlike many recent House speakers, the Cincinnati-area native is born of a legislative tradition. He chaired the then-House Education and Workforce Committee for years and helped craft No Child Left Behind, working with the late Democratic Sen. Edward M. Kennedy of Massachusetts.