The size of House Democrats' shrinking majority comes down to nail-biter races

Democrats in the House might still have a majority to wield over the GOP next year, but their advantage is growing narrower as Republicans take minuscule leads, currently under 400 votes, in some of the final races of the season.

The drama surrounding the House contests has largely been eclipsed by President Donald Trump's relentless assailment of the country's electoral system, including his efforts to cast doubt over the integrity of the election and his campaign's string of unsuccessful lawsuits to challenge the results.

Despite President-elect Joe Biden clinching the White House, Republicans trimmed House Speaker Nancy Pelosi's 35-seat majority to just 12. With an additional two races outstanding in Iowa and New York and a runoff in Louisiana, current ABC News projections put the partisan breakdown at 222-210 -- leaving Democrats with one of the smallest majorities in two decades.

On Monday evening, Republicans scored another victory when Christy Smith, the Democratic assemblywoman in California, conceded to Republican Rep. Mike Garcia in the 25th Congressional District -- in what was one of the last remaining House races along with contests in Iowa and New York.

The margin separating the two rivals was 333 votes, which was far closer than the May special election earlier this year when Garcia first won the seat that formerly belonged to Democratic Rep. Katie Hill.

"Though I'm humbled by every vote we earned, the results show our district is deeply divided," Smith said in a statement.

Smith's concession closes out a contentious campaign, in which both candidates took the lead in turns over the nearly four weeks since Election Day.

Garcia, a former Navy fighter pilot, has now earned the seat outright for a two-year term -- returning it to reliably red after voters in the longtime GOP-held district, which covers northern Los Angeles County, backed Hillary Clinton in 2016 and Hill in 2018.

In the outstanding Iowa and New York races, the margins are razor-thin, while Louisiana’s 5th Congressional District is headed to a runoff on Dec. 5 between two Republicans, Luke Letlow and state Rep. Lance Harris, to replace outgoing GOP Rep. Ralph Abraham.

Republican Mariannette Miller-Meeks holds a six-vote lead at the end of a recount in Iowa. The state canvassing board certified the results on Monday after all 24 counties in the 2nd Congressional District completed the recount.

The official tallies put Miller-Meeks ahead of Democrat Rita Hart by one of the closest margins in history, 196,964-196,958.

"This race reinforces that every single vote can make a difference," said Secretary of State Paul Pate in a statement announcing the certification of the results.

There have been seven races throughout the country's history where candidates were separated by just one vote. The Iowa race is the closest race for the House since 1984, when Democrat Frank McCloskey prevailed in Indiana's 8th Congressional District by just four votes over Republican Rick McIntyre after several recounts.

But the race for the southeastern Iowa district is far from over.

Hart could either concede the race or further contest the election in the courts. She is required to take legal action within two days of Monday's certification, according to the Des Moines Register.

Her campaign manager Zach Meunier suggested after certification that they will continue their challenge.

"Over the next few days, we will outline our next steps in this process to ensure that all Iowans' voices are heard," he said.

If Miller-Meeks is declared the winner, she will replace Democratic Rep. Dave Loebsack, who is retiring after seven terms in the chamber and her victory would further slash Democrats' majority in the House.

The party could also potentially lose a seat in upstate New York, where incumbent Democratic Rep. Anthony Brindisi is currently trailing his Republican challenger Claudia Tenney by 12 votes in a messy post-election.

The first official tallies from all eight counties in the district were submitted to a state Supreme Court judge on Monday. New York state's certification deadline is not until Dec. 7, one day before the "safe harbor" date, a deadline for states to certify results in order for their electoral votes to be insulated from challenges when presented to Congress.

As the race makes its way through the courts, it could be months before this contest, along with the one in Iowa, are resolved.

These nail-biters are expected to hold significant influence over Democrats' effectiveness over the next two years, with leadership facing an increasingly tougher road ahead to enact their agenda in the next Congress.

"The margins are infinitesimal but could make a huge difference in how the House operates," Dave Wasserman, the House editor for the Cook Political Report, wrote on Monday before Smith conceded in California. "These two races will likely decide whether Democrats have a 222 or 224 seat majority and whether Speaker Nancy Pelosi can afford four, five or six defections and still pass legislation."

ABC News' John Parkinson contributed reporting.