LONDON—While British Prime Minister Theresa May was visiting European leaders to find support for improving her deal for Britain’s exit from the European Union, some of her own party’s members of Parliament were preparing a no-confidence vote against her leadership.

On Wednesday, the process came to a head.

Here is a guide to the vote.

What Set Off the Vote

At least 15 percent of her party’s lawmakers — at least 48 members of Parliament — had to submit letters demanding a ballot to the chairman of the 1922 committee, the body that represents Conservative backbenchers.

The chairman, Graham Brady, announced on Wednesday that he had received more than the required number of letters, and that the no-confidence vote would proceed.

When Is the Vote Taking Place?

Conservative lawmakers were gathering in Westminster on Wednesday evening to give May a chance to address them.

After the prime minister’s address, lawmakers will cast ballots in a metal box for about two hours, from 6 to 8 p.m. local time, with the result expected within the hour.

May has to win 50 percent of the votes plus one, and there are 315 Conservative lawmakers.

More than the required number of lawmakers — over 158 — have publicly pledged their support, according to British news outlets and posts on social media.

Because the ballots are cast in secret, it is not certain that everyone who promised support for May will necessarily vote for her.

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What Happens If May Loses?

May will no longer be the leader of the Conservative Party, and Brady will begin the process to choose a successor, most likely on Tuesday.

Lawmakers will vote in rounds of secret ballots, with the least-popular candidate eliminated each time, until two contenders remain.

Brady said he believed that part of the process could be done by the end of business next Thursday, when Parliament is scheduled to break for the Christmas vacation.

From the two top candidates, the final choice would be left to around 120,000 members of the Conservative Party who would vote by postal ballot.

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The timing for that process, and the duration of any campaign before the vote, is unclear. In 2005, the last time the Conservatives held such a postal ballot, it took around six weeks.

And If She Wins?

The lawmakers in May’s party would be barred from challenging her for one year, which would strengthen her position.

A very narrow win could put her under pressure to resign, because dozens of the lawmakers voting are also government ministers, meaning she would have won only with the support of politicians paid to support her administration.

In 1990, one former prime minister, Margaret Thatcher, resigned after she was challenged under a different procedure and won only narrowly in a first round of voting.

May could also lose power after winning this no-confidence vote if Parliament as a whole passes a vote of no confidence in her government.

There is no sign of that happening yet.

Read more:

British PM Theresa May says she will fight no-confidence vote ‘with everything I’ve got’

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