“There’s so much polarization that they don’t even try to compromise anymore, a bizarre state of affairs for an institution that has been traditionally known for debate, deliberation and compromise,” said Joshua C. Huder, a senior fellow at Georgetown University’s Government Affairs Institute.

“If we don’t see something happen in the next couple months, then you can probably expect this Congress not to do much policywise, other than a budget deal and a couple appropriations bills,” he added. “And that won’t get done on time.”

But Mr. McConnell, for his part, appears to have little concern about the lack of legislative agenda and has proudly fashioned himself as the “grim reaper” of progressive policies, first before a small group of constituents in Kentucky last month and then before a national stage.

“As long as I am majority leader of the Senate, I get to set the agenda,” Mr. McConnell said last week in an interview on Fox News, outlining his opposition to some of the most prominent progressive policies up for debate among the Democratic caucus. “That’s why I call myself the grim reaper. We will not have the Green New Deal and we will not have ‘Medicare for none’ as long as I am majority leader of the Senate.”

Democrats have begun to hurl the sobriquet back at Mr. McConnell, who is up for re-election in 2020.

“Senator McConnell has described himself as the grim reaper; he’s going to kill every bill that comes over from the House,” Speaker Nancy Pelosi of California said, slamming Republican inaction at an event on Wednesday meant to promote her party’s health care legislation. “The Senate is going to be hearing from the American people.”

Other experts who analyze the productivity of the legislative branch emphasized that the Senate still had time to outline an agenda — and that it was part of a trend that had merely accelerated under Mr. McConnell, particularly with a shorter debate time over nominees.