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 — refashioning the country’s social safety net, wiping out scores of labor and environmental regulations and unraveling some of the most significant policy prescriptions put forward by the Obama administration.
Even before President-elect Donald J. Trump is sworn in on Jan. 20, giving the G.O.P. full control of the government, Republicans plan quick action on several of their top priorities — most notably a measure to clear a path for the Affordable Care Act’s repeal. 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 Mr. 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, Mr. 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 cleave 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.
“They’ve been given a golden opportunity here,” said Trent Lott, the former Republican Senate majority leader. “But I have watched over the years when one party has had control of the White House and the Senate and the House, and the danger is overplaying your hand. If you go too far like what happened with Obamacare, and you get no support at all from the other side, you have a problem. You have to find a way to work with people across the aisle who will work with you.”
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 Mr. Trump’s chief of staff, Reince Priebus.
The Senate may be narrowly divided, but among the 48 Democratic senators are 10 who will stand for re-election in two years in states that voted for Mr. 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 Mr. 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, Democrat of Washington. “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 Mr. 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 D. 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 the practice of mountaintop-removal mining. That regulation, one of dozens that Mr. 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 current 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 Mr. Obama’s, and they may delay the repeal measure’s effective date for years.
The Senate must also confirm Mr. Trump’s cabinet picks, and Senate Democrats are already trying to slow the process. However, they cannot do much more than that, because, when they were in charge, they changed the rules so that presidential nominees other than Supreme Court picks need only 51 votes to be confirmed. Previously, such nominations could be filibustered, which required 60 votes to overcome.
Democratic leaders have encouraged members to avoid meeting with Mr. Trump’s nominees until they have turned over their tax returns and made other disclosures. Republicans have been particularly upset that Senator Jeff Sessions of Alabama, whom Mr. Trump picked quickly to be attorney general, has either not gotten meetings with Democrats on the Senate Judiciary Committee or had meetings canceled.
Senator Dianne Feinstein of California postponed her meeting with Mr. Sessions until January because, she said, her schedule got too busy. “The senator doesn’t want to rush,” said her spokesman, Tom Mentzer. One reason that Democrats are in no hurry is their bitterness over Mr. McConnell’s refusal last year to hold a hearing on the Supreme Court nomination of Judge Merrick B. Garland.
Lingering in the background is the specter of Russia. Democrats — and some Republicans, who are at odds with Mr. 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.
While Republicans may have a rare chance to open the flow of legislation, the party’s leaders are acutely aware of the punishment Americans have historically delivered in midterm elections when things have not gone well. “This is no time for hubris,” Mr. McConnell said. “You have to perform.”