While May’s downfall has been predicted many times since she assumed the top U.K. executive role in 2016, this is the first time enough private and public opposition appears to have materialized to force her ouster, including key party figures openly stating their lack of confidence in her leadership.

Unless a majority of Tory members of Parliament (MPs), who number 315 or 48.5% of the chamber, pass a vote of confidence, she must resign as leader of the party. Loss of that role results in her ouster as prime minister as well.

That loss leads to a leadership election by the Tories from which May is excluded by party rules. The winning member becomes prime minister without a general election. A prime minster is also an MP, and retains her or his parliamentary position as the de facto head of government so long as they retain control of the governing party.

This is distinct from a no-confidence vote in Parliament, in which the government in power fails to achieve a majority of all members on certain motions, or an opposition motion expressing censure receives a majority. In both cases, the government must resign or call an immediate “snap” election. Such efforts are rarely successful, prevailing previously most recently in 1979 and 1924.

Among opposition parties, the Labour Party (with 257 seats) has hinted at issuing such a parliamentary motion, while the Scottish National Party, the next largest with 35 seats, has openly stated it would support one.

The most recent time a party changed leadership without a general election, May succeeded David Cameron, who resigned in 2016 after betting that a majority of U.K. citizens would vote in favor of remaining in the European Union and against the U.K.’s exit from the E.U., known as “Brexit.” However, a small majority chose the “leave” option in the vaguely stated non-binding referendum. May was elected by her party as leader.

The 1922 Committee, a group within the Tory Party, has the power to trigger a party vote of no confidence if at least 48 members submit private letters asking for the vote. Several reports indicate that as many as 53 letters have been received, including from some who have publicly denounced May’s leadership. If a vote is called, it is unlikely May would receive the necessary majority vote, and the Tories would elect a new leader.

May has held tenaciously on to power following a disastrous general election she called for June 2018 that caused the Tories to lose its majority in Parliament, and required it to form a coalition with the hard-right unionist Protestant party in Northern Ireland that holds just a few seats and extreme positions relative to the Tories.

She continued to keep her position as prime minister during lengthy and contentious negotiations with the E.U. over “Brexit,” despite a lack of clarity during most of the time as to what a final Brexit would look like. Both successive cabinet members in charge of negotiating Brexit have resigned.

However, opposition to May has reached a fever pitch in recent days, as an agreement on terms for Brexit with the E.U. failed to meet the demands of “hard” Brexit backers, who want a complete split, while retaining nearly full opposition from “Remainers,” who never wanted to leave the E.U. in the first place.