Democrats began the last Congress with the smallest House majority in 20 years: just 222 seats, four more than the minimum required for control.
In January, when the next Congress begins, Republicans could be back in power in the lower chamber -- with an even smaller majority.
As of Monday night, ABC News estimates that the GOP won 214 House seats in last week's midterm elections. There are 14 districts still unprojected, though Democrats lead in some, and FiveThirtyEight forecasts that Republicans may retake the House with as few as 218, which is the minimum needed.
That has never happened in the modern era and already has some conservatives wondering how governable their caucus would be with such limited room for error.
"Tread carefully," one House Republican, granted anonymity to speak candidly, said when asked what advice they'd give GOP leader Kevin McCarthy, who is seeking the speakership. "This is just such a divisive time. It's divisive within our party, it's divisive Republican-on-Democrat and vice versa."
The makeup of every Congress is fluid at the margins. At various points since January 2021, for example, there have been four, five, six -- as many as seven -- simultaneous vacancies in the House. Lawmakers die, resign or leave office for other positions in government. Those changes could have day-to-day ramifications on a majority of only a few seats.
McCarthy and other Republicans entered the midterms bullish on flipping as many as two dozen districts in the House. Anticipating a return to power, McCarthy had unveiled a "Commitment to America" while promising to focus on immigration at the southern border, cutting funding to the IRS, reassessing U.S. support for Ukraine and investigating the Biden White House.
But last week's election results indicate that McCarthy would instead have to lead with few, if any, votes to spare on those priorities from a fractious conference made up of competing viewpoints from moderates, pro-Trump representatives, fiscal conservatives and more.
"It's going to be very difficult to navigate and keep everybody happy, but he cannot make everybody happy," the House Republican said.
Former Rep. Mike Bishop, R-Mich., put the lack of a large margin another way. "It makes a big difference for a leader to have that kind of cushion," he said.
In a Sunday appearance on ABC's "This Week," current House Speaker Nancy Pelosi was asked by George Stephanopoulos how she sees McCarthy managing his party if he is in control.
Pelosi noted that Democrats currently maintain a five-seat majority and have been successful in passing legislation.
"It depends on their purpose. In our House, we had those kinds of numbers. But we were united," she said.
Would divisions hamstring moves on Ukraine, Biden?
The Republicans who spoke with ABC News for this story had said that if there is a GOP majority, McCarthy would be expected to start the new Congress with goodwill, given his financial and political support for many other members and his alliances with the prominent pro-Trump right flank.
After the midterm results became clear last week, however, some GOP lawmakers began openly voicing discontent.
Still, it remains unclear who else in the caucus would be able to challenge him. Others in leadership, like Steve Scalise, are seeking lesser positions in the intraparty election on Tuesday. (The official vote for speaker won't be until January.)
But if he remains atop a small Republican majority, McCarthy would likely be forced to dive headlong into a spate of policy battles both with the White House and internally, GOP lawmakers and operatives said.
The previous two Republican speakers, Paul Ryan and John Boehner, both retired after running the House. Boehner specifically cited a desire to avoid the "turmoil" of leadership jockeying.
Already, a debate over Ukraine aid has risen, with McCarthy last month warning that there should be no "blank check" from the U.S. to support the country against Russia's invasion. He later insisted that didn't mean there'd be no assistance at all, saying on CNBC, "Wouldn't you want a check and balance in Congress? Wouldn't you want this hardworking taxpayers' money, someone overseeing it?"
"I'm very supportive of Ukraine," McCarthy told CNN last week. "I think there has to be accountability going forward."
The issue has divided Republicans. Controversial Georgia Rep. Marjorie Taylor Greene vowed earlier this month that "under Republicans, not another penny will go to Ukraine. Our country comes first."
Greene, a popular figure among Trump supporters, has also boasted openly of what she called her influence with Republican voters. She previously suggested to The New York Times that "to be the best speaker of the House and to please the base, [McCarthy is] going to give me a lot of power."
McCarthy has promised to restore Greene to congressional committees after Democrats and some Republicans stripped her of her assignments last year, citing her history of inflammatory statements.
On Monday, Rep. Matt Gaetz, R-Fla., insisted in an appearance on Charlie Kirk's show that he and four other Republicans would try to block McCarthy in the leadership race. "Every five people is essentially a veto now," he said.
Elsewhere on Monday, Greene signaled on embattled former Trump political adviser Steve Bannon's podcast that she supported McCarthy and disagreed with a possible challenge to him from other conservatives in the party. She cited how few seats they may hold in total.
"I actually think that's a bad strategy when we're looking at having a very razor-thin majority with potentially 219. We're talking about one vote," she said.
She told ABC News later Monday that "I think Republicans need to pick our speaker and not allow the Democrats to pick our speaker."
Scalise also reiterated that McCarthy "will be the speaker."
"Who else would it be?" Texas Republican Rep. Dan Crenshaw told ABC News on Monday.
Domestically, McCarthy could use the federal debt ceiling -- which must be raised by Congress to cover the government's financial obligations -- as leverage to obtain concessions on spending, pleasing conservatives but risking a default on the debt. Similarly tough battles are anticipated to play out over funding the government.
"We've seen a willingness to stress test it in the past. But they'll prevent default," former Rep. David Jolly, R-Fla., said of the debt limit, which Democrats are expected to attempt to raise in the lame-duck session before January. "The world markets will not allow Kevin McCarthy and the House Republicans to take us to default. But shutdown of government? Yeah, I think we can expect a couple of shutdowns over the next two years."
Jolly also predicted little legislative consensus with President Joe Biden, whom many Republicans feel they were elected to challenge.
When asked if there's room for larger cooperation, Jolly replied, "I would say only on the budget."
Biden said Wednesday that he would invite Republican leadership to the White House, and he and McCarthy spoke the same day. But White House press secretary Karine Jean-Pierre offered few details to reporters on areas of possible collaboration. She said Thursday that Biden would not "renegotiate new things that have already passed."
Beyond policy, McCarthy would likely face pressure from lawmakers like Greene and others who advocate for an adversarial oversight effort into the Biden administration.
Greene has been vocal about impeaching Biden over a range of policy issues, while other Republicans have foreshadowed investigations of the president's son Hunter Biden -- as well as immigration, COVID-19, outgoing medical adviser Dr. Anthony Fauci and more.
"I don't care whether you're a Republican or Democrat, but if there's wrongdoing there, it deserves to be looked at, and I think the American people deserve to be told the truth about that," said one former House GOP aide of Hunter Biden, who has said he did nothing wrong with his business dealings.
This former aide said there was a risk, however, of investigations becoming their own distraction: "That cannot be the Republicans' main priority. That cannot be one thing where we retake the House, and then we have Kevin McCarthy coming out here and saying, 'OK, well, our No. 1 priority is going to be investigating Hunter Biden.' No, because then you're going to turn a lot of people off."
A GOP pollster spoke even more bluntly. "It would show amazing restraint to not do the stupid s---," they said. "Focus on the issues, focus on taking down the administration from a policy standpoint, focus on things that matter to people. You know what doesn't matter to people? Impeaching Biden. You know what doesn't matter to people? Hunter Biden, relitigating Fauci. That kind of stuff's stupid."
McCarthy recently told CNN that "we will never use impeachment for political purposes. That doesn't mean if something rises to the occasion, it would not be used at any other time."
Don't ignore the moderates
Some of the Republican operatives said that McCarthy, as speaker, would also have to pay attention to House moderates. Multiple of the party's few midterm successes in Congress came in districts in reliably blue New York.
One such lawmaker, Mike Lawler, struck an amenable tone in victory remarks. "I can assure everyone that I will do the best to represent all of the people from every community, from every walk of life, whether you voted for me or not and whether or not you agree with my politics," he said.
"Oftentimes, the moderates who behave, they're the ones who kind of get left out in all this. It's a balancing act," one GOP strategist working on House races told ABC News.
Still, McCarthy has relatively fewer options to clamp down on dissent if he ruffles feathers.
"Twenty years ago, you could say, 'Hey, we're gonna cut off your money or we're going to cut off your communications, your press.' But in today's media environment, you can't do that kind of stuff," the pollster said. They pointed to Greene: "[She] is still gonna have a microphone."
The strategist said little was certain about the months to come, including the trajectory of a McCarthy majority.
"He will be the speaker on day one," they said. "How long that lasts is anyone's guess."
ABC News' Katherine Faulders, Jay O'Brien and Allison Pecorin contributed to this report.