WASHINGTON — The most powerful and ambitious Republican-led Congress in 20 years will convene Tuesday, with plans to leave its mark on virtually every facet of American life.

It plans to refashion the country’s social safety net, wipe out scores of labor and environmental regulations, and unravel some of the most significant policy prescriptions put forward by the Obama administration.

Even before President-elect Donald Trump is sworn in Jan. 20, giving their party full control of the government, Republicans plan quick action on several of their top priorities. Most notable is a measure to clear a path for repeal of the Affordable Care Act, although the repeal is not expected to take effect for years because Republicans are divided on how to replace the program.

Perhaps the first thing that will happen in the new Congress is the push for deregulation. Also up early: filling a long-vacant Supreme Court seat, which is sure to set off a pitched showdown, and starting confirmation hearings for Trump’s Cabinet nominees.

“It’s a big job to actually have responsibility and produce results,’’ said Senator Mitch McConnell of Kentucky, the majority leader. “And we intend to do it.’’

But as Republicans plan to reserve the first 100 days of Congress for their more partisan goals, Democrats are preparing roadblocks.

The party’s brutal election-year wounds have been salted by evidence of Russian election interference, Trump’s hard-line Cabinet picks, and his taunting Twitter posts. (On Saturday, he offered New Year’s wishes “to all,’’ including “those who have fought me and lost so badly they just don’t know what to do.’’)

Obstacles will also come from Republicans, who are divided on how to proceed with the health care law and a pledge to rewrite the tax code.

Some are also skittish about certain policy proposals, like vast changes to Medicare, that could prove unpopular among the broad electorate. And any burst of legislative action will come only if Congress can break free of its longstanding tendency toward gridlock.

For Republicans, the path to this moment has been long and transparently paved — the House in particular has signaled the Republican policy vision through bills it has been passing for years. But many of those measures have gathered dust in the Senate or been doused in veto ink.

The cleft between the two chambers recalls the situation faced by the insurgent House Republican majority in the mid-1990s. Speaker Newt Gingrich took control with a determined agenda, only to be stymied by the Senate majority leader, Bob Dole, who stacked conservative House bills like so many fire logs in the back of the Senate chamber.

The tax overhaul and an infrastructure bill may be two opportunities for bipartisan cooperation — the Senate Finance Committee is already moving in that direction. Still, both of those issues are expected to remain on the back burner, despite promises to the contrary from Trump’s chief of staff, ­Reince Priebus.

The Senate may be narrowly divided, but among the 48 senators in the Democratic caucus are 10 who will stand for reelection in two years in states that voted for Trump. Republicans are counting on their support, at least some of the time.

But on many issues, Senate Democrats — including their new leader, Chuck Schumer of New York — are expected to pivot from postelection carping to active thwarting, using complex Senate procedures and political messaging to slow or perhaps block elements of Trump’s agenda.

“After campaigning on a promise to help the middle class, President-elect Trump’s postelection actions suggest he intends to do the exact opposite after he’s sworn in,’’ said Senator Patty Murray, a Washington Democrat. “Democrats will do everything we can to fight back if he continues to pursue an agenda prioritizing billionaires and big corporations while devastating middle-class families and the economy.’’

Republicans have chafed for years at a host of rules, many business-related, that Obama has issued through the regulatory process, and they have been advising the Trump team on which ones should be undone.

“I hear probably more about the strangulation of regulations on business and their growth and their development than probably anything else,’’ the House speaker, Paul Ryan of Wisconsin, said at a recent forum. “I think if we can provide regulatory relief right away, that can breathe a sigh of relief into the economy.’’

In late December, the Obama administration rolled out a major new environmental regulation intended to rein in mountaintop-removal mining. That regulation, one of dozens that Trump is expected to reverse, is meant to go into effect one day before his inauguration.

But Congress is likely to block it, using the obscure Congressional Review Act, which permits lawmakers to undo new regulations with only 51 Senate votes within the first 60 legislative days of the rules’ completion.

Given time constraints on the Senate floor, members will have to pick some priorities. They are expected to train their sights on a rule that requires oil and gas producers to reduce methane gases, another that requires mining and fossil fuel companies to disclose payments they have made to foreign governments to extract natural resources, and still others that restrict pesticide use.

Republicans will also move quickly to repeal the Affordable Care Act. They plan to pass a truncated budget resolution for the remainder of the fiscal year — already a quarter over — that includes special instructions ensuring that the final repeal legislation could circumvent any Democratic filibuster.

But Republican leaders have not settled on a health care plan to replace Obama’s, and they may delay the repeal measure’s effective date for years.

Lingering in the background is the specter of Russia. Democrats — and some Republicans, who are at odds with Trump on the issue and may at times be a brake on him — want a vigorous investigation of its efforts to disrupt the election. The Obama administration, which took sweeping steps last week to punish the Russians over election hacking, will release a report this month that is likely to serve as a turning point in those discussions.