House Speaker Kevin McCarthy is overseeing one of the narrowest congressional majorities in decades – tied with two other time periods for the fifth-smallest majority in US history, according to the Pew Research Center. With control of 221 seats, McCarthy’s party controls just 9 more seats than Democrats’ 212, meaning that five defections are enough to sink any effort the GOP wants to push through – though that exact margin can change based on how many lawmakers are present and voting.

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In the past, political moderates have represented some of the most likely lawmakers to buck their party and flip their votes. However, in an increasingly polarized Washington, McCarthy’s majority has been marked by multiple high-profile votes where the far right of his party has blocked key efforts from advancing – leading the California Republican to make concessions to conservatives in exchange for their support.

Now, McCarthy’s narrow majority – and emboldened House conservative hardliners – look poised to take center stage as the threat of a government shutdown looms.

As the House returned from August recess, McCarthy called for the launch of an impeachment inquiry into President Joe Biden, which has been a key priority for many in the far-right of his party. Some Republicans told CNN they had hoped opening an inquiry could give McCarthy cover to cut a deal to keep the government funded beyond the September 30 deadline when funding is set to expire. But many conservatives have said publicly they want to keep the issues of impeachment and government funding separate and are threatening to vote against an effort to pass a short-term funding extension at the end of the month.

McCarthy has struggled to unite his conference behind his speaker bid from the start of this Congress. In January, it took 15 rounds of voting over multiple days for him to nail down the votes needed to win the gavel. The early rounds of voting offered a snapshot of the members of McCarthy’s conference – mostly from the right side of the party – who opposed the California Republican from the beginning.

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One of the highest profile fights on Capitol Hill this year was the fight to lift the nation’s borrowing limit. McCarthy struck a deal with President Joe Biden in May to raise the US debt ceiling, but he angered conservatives who have made demands for deep spending cuts and were not happy with the deal. The end result meant that McCarthy needed to rely on Democratic votes to pass the measure 314-117. There were 149 Republicans and 165 Democrats who voted for the bill and 71 Republicans and 46 Democrats who voted against it.

In a key procedural vote leading up to the passage of the debt ceiling deal, 29 House Republicans voted in opposition – a group made up of frequent McCarthy critics. McCarthy needed Democrats to help advance his bill toward final passage.

These three votes offer a snapshot of the conservative opposition facing McCarthy – a bloc of members that can shift and change depending on the decisions he makes. As the speaker works through a high-profile and divisive series of issues, he could flip opponents to supporters as well as alienate allies.

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McCarthy has the freedom to work with Democrats when he needs votes to pass key priorities, but any vote that relies on the opposing party threatens to undermine the power the House speaker has over his conference. Concessions McCarthy made to House rules in January in order to win his speakership now allow any member of the conference to call for and force a vote for House speaker. Conservatives have threatened to oust McCarthy if he doesn’t commit to their demands over impeaching Biden as well as spending cuts.

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With such a tight margin, any vacancies or absences can affect the total majority needed to pass a vote. One safely Democratic seat – Rhode Island’s First Congressional District – is open after Rep. David Cicilline stepped down. Rep. Chris Stewart announced plans to resign Congress by September 15, chipping one vote away from the House GOP conference. That safely Republican seat is expected to be filled following a special election in late November.

With the expiration of Covid-19-era provisions that allowed for proxy voting, House members must vote in person. This means that any lawmaker absences can raise or lower the threshold for a majority, a calculation party whips – and McCarthy in particular – need to keep in mind.