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 — 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 their party 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.”)