Some of Tuesday’s US midterm races may take weeks to clear recounts and court challenges, but ultimately the Senate and House of Representatives decide who gets seated, a power that could matter if a handful of contests determine control of Congress.

Federal elections are run by states and certified by state officials. But the US Constitution makes each chamber of Congress "the judge of the elections, returns and qualifications of its own members," giving lawmakers the final say when a House or Senate result is contested.

Nearly six years after supporters of President Donald Trump attacked the US Capitol on January 6, 2021, as Republicans sought to overturn his 2020 loss to Democrat Joe Biden, experts say competitive races could be close enough to trigger challenges, starting at the state level, as the two parties jockey for control of Congress.

"Those within the recount or contest margin of error – the candidates, outside groups and the administration – will fight those tooth and nail," said Victoria Bassetti, senior advisor at States United Democracy Center, a nonprofit group focused on protecting free, fair and secure elections.

Election litigation has surged since the 2000 US Supreme Court ruling that resolved the presidential contest between Republican George W. Bush and Democrat Al Gore. A sampling compiled by UCLA law professor Rick Hasen found a record 756 cases during the 2023-2024 election cycle, up 14.3% from 2019-2020 and roughly triple the level before 2000.

The results of congressional elections can be challenged in both the House and the Senate by candidates and sitting members. In the House, a losing candidate can challenge a state-certified result under the Federal Contested Election Act. The House Administration Committee considers the challenge and can recommend to the full House who should be seated.

In the Senate, challenges are referred to the Senate Rules Committee, which reviews cases and makes recommendations to the full Senate. But floor fights can occur. If, say, a sitting Republican planned to object to the seating of a Democrat and filibuster the issue, Democrats could respond tit-for-tat or boycott the chamber to deny it the quorum to convene.

Any House contests still unresolved when the new Congress convenes in January would fall to the new House, not the outgoing one. That could complicate control of the chamber if enough races remain unsettled. The new House must also organize and elect a speaker. Partisan gridlock has sometimes left contested seats vacant for extended periods.

"I don't think it's a good idea for people to get panicky or alarmist about it," Michigan State University law professor Brian Kalt said. "But after January 6, it would be naive to say there's no potential for shenanigans." Since the first Congress met in 1789, the House has grappled with 634 contested elections, according to its website. The Senate has had at least two dozen since senators were first chosen by direct popular election in 1914, Senate records show.

The House and Senate, which Republicans currently control by slim margins, could change hands if Democrats manage to pick up a net four seats in each chamber. All 435 House seats are up for election, while only about one-third of the Senate's 100 seats are.

The Trump administration has already intervened in this year's elections, suing to block Alaska from certifying results if Republican Senator Dan Sullivan loses and to prevent California from counting more than 20,000 mail-in ballots.

The House and Senate have each established the practice of seating candidates provisionally in contested races, while results are being litigated or investigated. But lengthy court contests can delay certification and leave seats vacant.

No contested election has determined control of the Senate in recent history, but there have been some close calls since 1914. With as many as seven races rated toss-ups this year, experts point to two examples in which Senate seats remained vacant for months.

In 2008, a Minnesota court battle between Democratic challenger Al Franken and Republican Senator Norm Coleman left the seat open for months. The case was finally resolved in Franken's favor in July 2009, six months after Coleman's term had ended.

A 1974 Senate election in New Hampshire went unresolved for even longer, with the Senate unable to decide the contest between Democrat John Durkin and Republican Louis Wyman. Separate state recounts declared different winners. State courts declined to get involved. A Senate election panel deadlocked. Debate dragged on until lawmakers declared the seat vacant in July 1975, allowing for a special election that Durkin won that September.

The House has had far more contested elections than the Senate. The nonpartisan election analysts at Inside Elections rate 18 current races as toss-ups, while still more appear to be competitive.

The House has averaged just over five contested races for every election since 1789, with most occurring during the 19th-century Civil War, Reconstruction, Gilded Age and early Jim Crow era. If Democrats win control of the chamber, experts say a large enough wave could make contested races irrelevant to the outcome.

In 1895, after Republicans swept to power in a landslide election driven by public anger over hard economic times, a record 38 House races were contested. As a result, many races remained undecided more than two years after the election, according to official House records.