Democrats are unlikely to flip the 30 seats needed to take control of the lower chamber, but significant losses would leave Republicans with a slimmer majority, and the House will likely remain divided over the basics of governing.
"We're in a much stronger position than anybody thought," House Majority Leader Kevin McCarthy, R-Calif., told reporters in September.
But Republicans were left scrambling by a Washington Post report earlier this month on a damning 2005 video showing Trump boasting of groping women without their consent — which he has defended as nothing more than "locker-room talk."
In the weeks since, Trump has been accused of sexual assault by at least 10 women (he denies their charges) and has refused to accept the results of the election unconditionally.
Democrats have launched a series of ads across the country — from Orange County, California, to northern Wisconsin — tying Republicans to Trump's hot-mic "Access Hollywood" comments.
"It's clear that House Republicans are directly tied to Donald Trump and cannot separate from his toxic campaign this late in the game," Democratic Congressional Campaign Committee spokeswoman Meredith Kelly said in an email to ABC News.
House Democrats' campaign arm has run dozens of ads in House races across the country.
Republicans are working hard to build a firewall. As of this month, Ryan has raised nearly $50 million to protect the GOP majority and has spent the month on a 17-state, more than 40-city campaign blitz. House Majority Leader Kevin McCarthy, R-Calif., has raised and given nearly $25 million to Republican candidates this cycle.
Republican groups, including the National Republican Congressional Committee, have started running ads linking Democrats to a Clinton White House, calling on voters to support divided government and prevent a Congress that would rubber-stamp a Clinton agenda.
The GOP resources, along with Republicans' efforts to distance themselves from Trump, could help limit losses in November.
But a continued Trump slide would be difficult for many Republicans to outpace, especially those from suburban districts with well-educated voters, according to David Wasserman, the House of Representatives editor at the nonpartisan Cook Political Report.
"The rule of thumb might be a decline of 1 point probably hurts Republicans by half that much in a generic ballot," he said.
Among likely voters, Trump trails Clinton 50 to 38 percent, according an ABC News poll released Sunday — his lowest level of support to date in ABC News/Washington Post surveys.
That poll found the share of registered Republicans likely to vote is down 7 points since mid-October, a trend that could hurt down-ballot candidates who need the support of moderate suburban Republicans and Trump-supporting conservatives alike.
GOP operative Brian Walsh said Trump could be depressing GOP turnout with his unsubstantiated claims that the election is rigged.
Still, Democrats would need a nearly clean sweep of competitive districts to take back the House. Just 15 races are tossups, according to an ABC News analysis of the House races. (An additional 24 lean Republican, and 15 lean Democrat.)
"You can't rule it out entirely, but I seen nothing to indicate that Democrats are on a wave," Wasserman said, estimating that Democrats could gain 10 to 20 seats.
"It would require a wipeout scenario, and that would require a large number of Republican voters staying home."
A slim majority could spell trouble for Ryan and other GOP leaders, who would be leading a conference with a higher percentage of hard-liners skeptical of compromising with Democrats on even the most routine pieces of legislation.
"We need to have a healthy majority and a strong majority," Ryan recently told The Wall Street Journal's editorial page. "If we have a razor-thin majority, then every vote can be problematic. Every vote on everything can be difficult."