By Clare Foran, Ethan Cohen and Haley Talbot, CNN

(CNN) — The 119th Congress will begin on Friday, ushering in a new era of Republican control in Washington that will start with a high-stakes leadership fight to pick the next House speaker.

Mike Johnson is vying to retain the gavel and has President-elect Donald Trump’s endorsement, but he faces tough vote math with the narrowest House majority in nearly 100 years, leaving little room for error.

Johnson can only afford a single GOP defection if every lawmaker shows up and votes, and one House Republican - Rep. Thomas Massie of Kentucky - has already said he won’t vote for him, while roughly a dozen more have not committed to supporting him.

Looming over the race is the question of what happens if the House has not yet elected a speaker by Monday, January 6, the day lawmakers are supposed to count electoral votes and finalize the results of the presidential election – a scenario that would put Congress into unprecedented territory.

How many votes Johnson needs to win

To be elected speaker, a candidate must win a majority of votes out of all votes cast. If all 435 members of the House vote, then a majority is 218 votes.

There is expected to be one vacancy when the House convenes for the vote. Former GOP Rep. Matt Gaetz of Florida has said he won’t take the seat he was elected to in the new Congress.

That leaves 434 members with 219 Republicans and 215 Democrats. If all 434 vote, it takes 218 to win.

Members can vote for anyone they want

There’s no rule that the speaker must be a House member.

Members can vote for anyone, regardless of if the name was put into nomination or if the individual is a member of the House. Members can also effectively abstain from voting at all by simply saying “present” when they are called on to vote.

The vast majority of lawmakers, however, will vote for their party’s leader.

It doesn’t always require 218 votes

While 218 is typically thought of as the magic number, it’s possible to be elected speaker with fewer votes.

If any members do not vote or vote “present” – which is not really a vote at all – the number needed to win can go down. That’s because absences and “present” votes do not count toward the overall total used to calculate the majority threshold.

For example, if one member votes “present” or does not show up to vote at all, then the total number voting is 433 and the majority threshold drops down to 217.

How the speaker election will unfold

The new Congress will start at noon on January 3, and the first major item of business in the House is to elect a speaker. This happens before members are even sworn in.

Before voting can start, candidates must be nominated. GOP Rep. Lisa McClain of Michigan is expected to give a nominating speech for Johnson, and Democratic Rep. Pete Aguilar of California is expected to give a nominating speech for House Democratic leader Hakeem Jeffries. But any member can nominate anyone, so additional candidates could be nominated.

During the vote, a clerk will call on members by name in alphabetical order and each will have the chance to say which candidate they support. This is also when members could opt to say “present” instead of voting for a candidate.

At the end, any members who did not vote will be given the opportunity to do so, and if any member wants to change their vote, they can.

There could be a floor fight with multiple rounds of voting

Johnson’s allies are projecting confidence he’ll be able to secure the speakership, but even they are bracing for the possibility that it could go into multiple rounds of voting, leading to a drawn-out floor fight.

In 2023, it took Kevin McCarthy 15 rounds of voting over multiple days to win the speakership.

The House will need to hold a second speaker vote if Johnson fails to win in the first round.

If that happens, the chamber could immediately move into a second round of voting or GOP leaders could push for a break to regroup. Members can also propose motions, like a motion to adjourn the House.

How many votes could Johnson lose and still be elected speaker

There are a variety of different scenarios under which Johnson could suffer GOP defections and still be elected speaker, either through a vote against him for a different candidate or “present” votes.

Based on the expectation that there will be 219 Republicans and 215 Democrats, if all Democrats vote for Jeffries, Johnson can lose at most either: one GOP vote for another candidate plus one GOP “present” vote OR three GOP “present” votes.

What happens to Trump’s election certification if there’s no speaker next week

Johnson allies have been warning that a drawn-out speaker battle could delay Trump’s election certification. Traditionally, without a speaker, the House can do nothing besides vote to elect a speaker, vote to recess and vote to adjourn.

Unlike the last time the House was without a speaker, there won’t be a temporary leader poised to take over quickly. Speakers are directed to submit a list of members to serve as speaker pro tempore – or temporary speaker – in the event of a vacancy. And when McCarthy was ousted, then-Rep. Patrick McHenry was at the top of his list. However, in this case, there would be no speaker and, as a result, no list.

But if Johnson can’t get the votes, it’s possible the House could elect a temporary speaker to guide the chamber through the certification process before deciding on a permanent leader.?That could be done either informally, by the election of a caretaker speaker who agrees to resign after the certification, or, some experts argue, more formally, by voting to appoint someone with a set, short term.

Another option that is being privately floated around Washington: Pushing the date of Trump’s certification. But there’s not much time to delay as January 20 is the date of inauguration.

If the votes aren’t counted by then, President Joe Biden and Vice President Kamala Harris would leave office, as mandated by the Constitution, which states their terms end at noon on that day – but neither Trump nor Vice President-elect JD Vance would be able to take the oath of office.

CNN’s Lauren Fox, Sarah Ferris, Manu Raju and Zachary B. Wolf contributed to this report.

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