After one of the closest contests in American history, the House must now decide whether to unseat Mariannette Miller-Meeks, a Republican.

Three months after its count of the presidential election results set off a riot at the Capitol, Congress has plunged once again into a red-hot dispute over the 2020 balloting, this time weighing whether to overturn the results of a House race in Iowa that could tilt the chamber’s narrow balance of power.

At issue is the outcome of November’s election in a southeastern Iowa district, where state officials declared Representative Mariannette Miller-Meeks, a Republican, the winner in one of the closest contests in American history. Ms. Miller-Meeks prevailed by only six votes out of nearly 400,000 cast in the state’s Second Congressional District; in January, she took the oath of office in Washington.

But her Democratic opponent, Rita Hart, has refused to concede the race, pointing to 22 discarded ballots she says would have made her the winner if counted. Now Democrats, who hold the majority in the House and spent months pushing back on President Donald J. Trump’s falsehoods about a stolen election — including his claim that Congress had the power to unilaterally overturn the results — are thrust into the uncomfortable role of arbiters of a contested race.

Ms. Hart has appealed to the House, including in a new filing on Monday, to step in to overrule the state and seat her instead, sending Ms. Miller-Meeks back to Iowa.

“This was not something I sought, believe me,” said Representative Zoe Lofgren, Democrat of California and the chairwoman of the panel looking into the race.

Ms. Lofgren and other Democrats say they have little choice but to take the appeal seriously under a 1960s law Ms. Hart has invoked. In recent weeks, Ms. Lofgren’s panel, the House Administration Committee, has opened a full-scale review into the contest that lawmakers say could lead to impounding ballots, conducting their own hand recount and ultimately a vote by the full House to determine who should rightfully represent the Iowa district.

Reversing the result would give Democrats a crucial additional vote to pad one of the sparest majorities in decades. The House is currently divided 219 to 211, with five vacancies.

That prospect has rapidly reignited tensions in a chamber that has scarcely begun to heal from the deadly Jan. 6 attack on the Capitol by a pro-Trump mob trying to stop Congress from formalizing President Biden’s victory. House Republicans — more than half of whom voted that day to discard state certifications and overturn Mr. Biden’s win — are accusing Democrats who ostracized them of a screeching, 180-degree turn now that flipping an election result would be to their advantage.

“One hundred percent, pure partisan politics,” said Representative Rodney Davis of Illinois, the top Republican on the Administration Committee. “It wasn’t too long ago that many of my Democratic colleagues were saying a certificate of election by state officials were sacrosanct.”

Mr. Davis moved unsuccessfully this month to dismiss the challenge, and his party’s political operatives are using it to assail Democrats and galvanize their own core supporters. Republicans, by accusing Democrats of trying to “steal” a seat to bolster their exceedingly narrow majority, believe they can stoke the anger of a base that believed Mr. Trump’s false claims that Democrats cheated in the 2020 election. They hope to drive a wedge between Democratic leaders who have agreed to consider Ms. Hart’s challenge and rank-and-file members from conservative-leaning districts who fear it could undermine their credibility with voters.

Democrats insist the charges are preposterous. The Administration Committee has merely agreed to hear the case, they argue, and Ms. Lofgren said in an interview that she had no idea what the panel might recommend. She called Republicans’ characterizations of her motivations “insulting,” but acknowledged she had a political headache on her hands — one that has made some of her own Democratic colleagues squirm.

Rita Hart, the Democratic challenger, has refused to concede a race she says was wrongly decided.
Rebecca F. Miller/The Gazette, via Associated Press

“The comments made by some of the Republicans — whether they are ignorant or malicious I can’t say, but they have nothing to do with the obligation the committee has,” she said.

The Constitution gives each house of Congress, not the states, the final say over the “elections, returns and qualifications of its own members,” and over the past century, the House has considered more than 100 contested elections. In 1969, Congress passed the Federal Contested Elections Act to set up a clear process governing how it should hear and decide the cases.

Actually overturning the results, though, has been exceedingly rare, happening in only a handful of cases. Lawmakers in both parties have shown a general preference to defer to state election laws and determinations wherever possible.

The contest between Ms. Miller-Meeks and Ms. Hart, both 65, appears likely to test whether Democrats want the body to wade into Iowa state election law and second-guess the state’s bipartisan certification.

Unlike Mr. Trump and many other officials who have made election appeals to the House, Ms. Hart is not claiming there was fraud at play in the result. Instead, her campaign has identified 22 ballots that they believe were legally cast but “wrongfully” uncounted by state election officials during a districtwide recount in the fall. Among them are ballots that were cast curbside by disabled people but not accepted by voting machines, one that was discarded because it was sealed with tape, another that was signed in the wrong place, and a few that simply were not included in the tallying because of clerical errors.

If they had been, Ms. Hart says that she, not Ms. Miller-Meeks, would have won the election by nine votes.

“Congress has an obligation to ensure not just that people have a right to vote, but a right to have their vote counted,” Marc E. Elias, Ms. Hart’s lawyer, told reporters on Tuesday. “Right now, at its core, we have 22 voters who have had their right to have their vote counted denied.”

Lawyers for Ms. Miller-Meeks say Ms. Hart’s complaint amounts to a disagreement with the judgment of bipartisan state election officials who decided which ballots to count. That, they argue, is simply not a good enough reason for the House to intervene, particularly after Ms. Hart declined to first press her case in Iowa state court last year before the contest was certified.

“The idea that the House would intervene is an extraordinary step,” said Alan R. Ostergren, a lawyer for Ms. Miller-Meeks, who has quickly earned a reputation as a rare moderate in her party. “Normally, a contestant would have to show fraud or irregularities. They would have to do more than she has done here, which is pointing out ordinary decisions about handling ballots and ordinary application of Iowa law.”

The fight could become costly. Democrats on the committee have already retained outside counsel from Jenner & Block, a firm based in Chicago, and Republicans have tapped Donald F. McGahn II, a former White House counsel and Republican elections lawyer, to advise them. The committee may also have to reimburse both candidates’ legal fees, which are currently being covered by each of their party’s campaign committees.

Mr. Davis and Republicans on the Administration Committee have also accused Democrats of a “serious conflict of interest” because Mr. Elias also represents several Democrats sitting in judgment of her case. Mr. Elias called it “nonsense.”

Speaker Nancy Pelosi has defended the House’s inquiry into the matter as routine business. But some Democrats, especially moderates from swing districts, appear increasingly uneasy and could shape the path ahead.

Representative David E. Price of North Carolina, a former political science professor, predicted on Sunday that there was not the “slightest chance” the House would follow through and overrule the state. Representative Chris Pappas, Democrat of New Hampshire, said it was “time to move on.” Others have warned their leaders not to try.

“Losing a House election by six votes is painful for Democrats,” Representative Dean Phillips, Democrat of Minnesota, wrote on Twitter. “But overturning it in the House would be even more painful for America. Just because a majority can does not mean a majority should.”