With the world mesmerized by the United States’ presidential race this year, the race for control of the US House of Representatives and Senate has been largely overlooked. But the outcome of the congressional elections could make or break the next president’s agenda.

For all the power a president has, the 100-member Senate determines the fate of international treaties as well as the president’s nominations and legislative proposals. The 435-member House does not have as much power as the Senate, but control of the White House, Senate and House by the same party could overcome much of the gridlock that has debilitated US governance in recent years.

The US does not have party-line elections, so voters can either punish a party (usually the one that has been in charge) by voting against all of its candidates; fully support a particular party; or split their tickets by voting for one party’s presidential candidate and another party’s congressional candidates.

Events have moved quickly since the Washington Post came upon an 11-year-old tape showing Republican presidential candidate Donald Trump bragging about his sexual aggressiveness. Not only have the Democrats’ chances to retake the Senate improved; so, too, have their prospects for retaking the House.

Previously, almost no one considered the House to be in play. Trump’s drop in the polls since the tape’s release has changed that, and the candidate has made matters worse for his party by denying that he has groped or assaulted women -- a claim that impelled numerous women to come forward and say otherwise.

Still, winning the House won’t be easy for the Democrats, because congressional districts have been heavily gerrymandered and Republicans control more of the governorships and state legislatures that oversee that process.

The Democrats must net at least 30 House seats and four Senate seats to retake control of each chamber, respectively. A net Democratic gain of four Senate seats would produce a 50-50 split, in which case, if Democratic presidential candidate Hillary Clinton wins, her vice president, Tim Kaine, would be on hand to break tie votes.

Senate candidates who have stood apart from Trump have been faring better than those who have not. For example, Trump was ahead in Ohio before the tape’s release; recent polling indicates that now he is trailing Clinton there. When the tape came out, Ohio Sen. Rob Portman (along with nine other senators) rescinded his endorsement of Trump, and he still seems to have a solid hold on his seat. Nonetheless, when Trump supporters attacked Republicans who had broken with him, a few re-endorsed him.

It is widely agreed that the top of a ballot influences races further down. Though the extent to which this happens isn’t clear, when it happens overwhelmingly, it is known as a “wave election,” as occurred in 1980 when Ronald Reagan trounced Jimmy Carter and helped the Republicans reclaim the Senate and 34 House seats. The Republicans didn’t have formal control of the House, but Reagan effectively had a working majority, because numerous Democrats from the South voted with them. While a lot may happen in the campaign’s remaining three weeks, the US could be heading toward another wave election.

Certain Senate races have long been considered in the bag for the Democrats. In Wisconsin, former Sen. Russ Feingold is decisively ahead of incumbent Sen. Ron Johnson, after having narrowly lost to him six years ago; and in Illinois, the GOP has effectively written off gaffe-prone Republican Sen. Mark Kirk.

Senate seats in New Hampshire, Nevada, North Carolina, Pennsylvania and elsewhere that were considered toss-ups before the tape’s release have come closer to being within the Democrats’ reach. Florida Sen. Marco Rubio, who ran against Trump in the Republican primaries, was considered likely to win reelection; now, he’s seen as vulnerable. If the US has a wave election, even Republican senators whose seats are still considered safe, such as Arizona Sen. John McCain, could be toppled.

But the Republicans could run interference against Clinton, even if they do lose in a wave. A Republican minority in the Senate can still effectively limit her agenda by using the filibuster, which requires 60 votes to end a floor debate and bring a bill to a vote. If Republicans keep control of the House, Speaker Paul Ryan, who may be eyeing the 2020 Republican presidential nomination, might try to cooperate with the president on some issues to show that he can get things done; but highly conservative House Republicans would likely rebel.

Meanwhile, the Supreme Court still has only eight members, instead of the usual nine, creating the possibility of split votes on key decisions. Republicans are anxious to keep the court’s ideological complexion intact after the death of the reliably conservative Antonin Scalia in February. That is why they have been blocking Obama’s nominee, Merrick Garland, since March.

So, even if the Democrats control the Senate starting in January, Republicans could still block a President Clinton’s Supreme Court nominations and policy program. And, because the Democrats are unlikely to win the House, the prospect for paralysis in Washington remains. 

By Elizabeth Drew

Elizabeth Drew is a regular contributor to the New York Review of Books and the author, most recently, of “Washington Journal: Reporting Watergate and Richard Nixon’s Downfall.” –Ed.

(Project Syndicate)