WASHINGTON — When a new Congress convenes in January under Democratic management for the first time since 1994, at least 20 of New England’s members of the House of Representatives will be Democrats.

At least four New Englanders in the Senate Democratic caucus will run major legislative domains, including Judiciary Committee Chairman Patrick J. Leahy of Vermont, with power to block any nomination to the Supreme Court.

And in Democratic National Chairman Howard Dean’s favorite illustration of his home region’s vanguard role in the party’s Election Day triumph: New Hampshire’s House went Democratic for the first time since the Buchanan administration.

All in all, the six-state region has enjoyed a power surge at the expense of that once-mighty breed of politician, the moderate Republican personified by such victims of President Bush’s unpopularity as Rhode Island Sen. Lincoln D. Chafee and veteran Connecticut Rep. Nancy Johnson.

“There is essentially no functioning Republican Party left in New England,” Dean, a former Vermont governor, said Thursday. “It’s the Republicans who did that themselves,” from the national level, because “they’ve been taken over by the far right.”

Republican National Chairman Ken Mehlman answered that portrayals of an all-blue New England are too facile. “Painting with that broad a brush is a mistake,” Mehlman said, disputing the notion that “Republicans can’t win in the Northeast.”

“In 20 years, one Democrat has been elected the governor in Massachusetts,” Mehlman said, referring to Deval Patrick’s election last week. He also pointed to Governor Carcieri’s reelection in Rhode Island and said Connecticut Republican Gov. Jodi Rell “swamped” her opponent on the way to reelection.

It’s also true, said congressional scholar Norman Ornstein of the American Enterprise Institute, that New England traditions of temperate politics will hold real power in the person of Connecticut Sen. Joseph I. Lieberman, who won reelection as an independent after losing a Democratic primary.

“Lieberman’s in the catbird seat” as possible leader of a bipartisan centrist group of as many as 10 senators — including Maine Republicans Olympia J. Snowe and Susan Collins — that could swing the vote on any given issue.

Connecticut is the region’s last Republican beachhead in the House. The powerful moderate Johnson lost outright. New London area Rep. Rob Simmons is slightly behind Democrat Joe Courtney in a contest undergoing a recount. That leaves fellow moderate Chris Shays as the only New England Republican certain to return to the new Congress.

Rhode Island’s two congressmen — both confined to the power-starved minority since they came to Washington — provide case studies of how the new Democratic majority may get results — or fall short — in the 110th Congress.

The House is so large and unruly — 435 members and many contesting factions — that it maintains order with rules that give the majority party near-total control over the agenda. Even though the Democratic leaders will have a cushion of only about 15 votes on a given roll call, they alone wield the power to decide whether a bill ever gets to the floor for a debate or a vote.

Rep. James R. Langevin’s experience typifies the frustration of the minority. Consider the debate over stem-cell research. Langevin has played an increasingly visible role in it since mid-2001. As a little-known freshman, he broke with other abortion foes to endorse the federal support of research into medical uses of stem cells derived from human embryos.

Mr. Bush decided in mid-2001 to block most such federal aid, objecting to the destruction of embryos that the research entails. Mr. Bush backs research into the use of stem cells taken from adult tissue. Langevin and other critics say that the adult stem cells do not hold as much promise for medical treatments as embryonic cells.

Republican House leadership, in tune with Mr. Bush’s opposition, blocked floor consideration of the issue for more than five years. But last year, under pressure from House Republicans mindful of the public’s mood on the stem-cell question, the leaders permitted a vote; an embryonic stem-cell bill passed the House.

The Senate passed a comparable bill, but Mr. Bush vetoed the eventual compromise. The legislation died because the House could not muster the super-majority needed to override a veto.

As the victim of a spinal cord injury that crippled him as a youth, Langevin is uniquely suited to make the political case for embryonic stem-cell research. Spinal damage is among the afflictions that might one day benefit from treatments arising from stem-cell research, so Langevin’s wheelchair is a powerful platform from which to speak for sufferers of many other ailments. In fact, the party featured Langevin’s stem-cell speech on the stage at the Democratic National Convention in 2004.

Inside the Congress, Langevin represents political “cover” for social conservatives of both parties who waver on embryonic stem-cell research. He underlines his stand against abortion by saying that his difficulties have enlivened his appreciation of human life. At the same time, he says, his handicap gives him a keen sense of the hope that stem-cell research holds out for eventual cures for millions.

Nothing about the argument has changed. But now that the Democrats are in charge, Langevin said he hopes to become an even more prominent voice in the stem-cell debate.

Langevin notes that the debate is coming soon. As Democratic speaker of the House, California Rep. Nancy Pelosi has pledged to bring a stem-cell bill to the floor as one of the top priorities of her “first 100 hours” of leadership — along with a higher minimum wage, a new set of ethics rules, and a bill to set in law the national security recommendations of the September 11 Commission.

The Democratic Senate should likewise have more than enough votes to pass an expansion of embryonic stem-cell research.

The new House appears to have even larger numbers of embryonic stem-cell research supporters than before — emboldened, now, by having won an election with that issue in the Democratic platform.

Langevin said it’s still not clear that the House would have enough votes to override a Bush veto. But, if not, the new political situation presents the Republicans with a stark threat in the next election cycle.

“We will make stem-cell research an issue in the 2008 campaign,” vowed Langevin.

Rep. Patrick J. Kennedy faces a somewhat trickier challenge in the pursuit of his signature legislation to force insurance companies to cover the mentally ill on an equal footing with the sufferers of bodily sickness and injury.

But mental health “parity,” as the insurance mandate is known for short, offers the intriguing prospect of a bipartisan Rose Garden signing ceremony that features Mr. Bush and the Kennedy team, father and son.

John Jonas, a prominent Democratic lobbyist who specializes in health-care issues, said before the election that the issue would hold much promise as a possible compromise area between Mr. Bush and a Democratic Congress.

“I think parity is good to go” under Democratic rule, in part because Mr. Bush has expressed support for the idea, Jonas said.

But general support from the president didn’t mean much when Congress was run by Republican leaders who never allowed House consideration of parity — despite some bipartisan support from membership.

Now Mr. Bush — and the Democrats — must face the complexities of imposing a major new burden on a large industry. Kennedy has long argued that the cost would be outstripped by the potential social savings, since mental illness and addiction impose huge costs on the nation’s medical, correctional and educational systems.

But Kennedy said in an interview Thursday that it will be important to proceed carefully on parity, with congressional hearings that consider the views of the health and insurance industries.

Partly for that reason, Kennedy said, parity is not on Pelosi’s legislative list of priorities for the first 100 hours of the new Congress. Nevertheless, Kennedy said that his work campaigning and raising money for Democratic candidates has won him much political capital with Pelosi and with the chairmen of two key committees that will consider parity legislation, Massachusetts Rep. Edward Markey and Michigan Rep. John D. Dingell Jr.

“I’ve got the House covered” in terms of the clout needed to push through parity legislation this year, Kennedy said.

On the Senate side, the Rhode Islander has a close ally on parity in the person of his father, Massachusetts Sen. Edward M. Kennedy, the incoming chairman of Health, Education, Labor and Pensions Committee.

As for Mr. Bush, Kennedy said mental health parity is “the kind of issue that would be perfect for him.” Politically, it would allow the president “to work in a bipartisan way for a public health legacy as well as a civil-rights legacy” that help a large group of Americans who suffer from a form of discrimination, Kennedy said.

Parity could also add to a Bush family legacy that includes the Americans With Disabilities Act that the elder President George H.W. Bush signed into law, Kennedy said.

Kennedy said the “coup de grace” is that Mr. Bush could sign legislation that “helps de-stigmatize an illness that he has grappled with himself, as do millions of other Americans.” Kennedy, himself a recovering addict and alcoholic, said he was referring to how Mr. Bush dealt earlier in his life with the problem of alcohol abuse.

Democratic Senator-elect Sheldon Whitehouse told reporters after his defeat of Chafee that he does not yet know what committee assignments he will draw. Like other Democrats, he has said a high priority will be to close the so-called “doughnut hole” — a gap in coverage for certain beneficiaries of the new Medicare drug benefit.

Incoming Democratic Senate Majority Leader Harry Reid of Nevada has said that he will make it a high priority to reduce drug benefit costs by letting Medicare seek bulk-purchase discounts from industry.

But Democratic health-care lobbyist Jonas is among those who warn that it will be “very difficult and very expensive” to close the coverage gap in the new benefit.