It's not unheard of for the U.S. president and Congress to not get along. But in the U.K., prime ministers live and die by how much political support they can command in Parliament.
That's because U.K. voters don't directly elect the prime minister. They only get to vote on who will be their local member of Parliament, or MP. The party with the most seats in Parliament then puts forward their leader as the next prime minister.
A prime minister who loses control of Parliament can face a vote of no confidence from rebellious MPs and be forced to step aside if they lose that vote. No-confidence votes can come in one of two ways: party leadership challenges or Parliament-wide votes.
The exact rules for leadership challenges differ in each party, but the process starts when a significant percentage of MPs band together to force a confidence vote. The prime minister then needs to secure support from at least half of his or her party's MPs to stay on as party leader.
If the prime minister loses that confidence vote, they're expected to stand aside while the party elects a new leader. A similar internal vote elevated Theresa May to be U.K. prime minister after previous Prime Minister David Cameron resigned in 2016.
But even if they weather the party confidence vote, prime ministers still need support from Parliament as a whole. Leaders of opposing parties can also bring up a confidence vote to test whether the current government has Parliament's support.
Those votes tend to be easy to win if the prime minister is backed by their party. But if enough rebel MPs vote no confidence alongside the opposition, the prime minister could lose their mandate to govern.
That's only happened once in the U.K.'s modern history: In 1979, Conservative Margaret Thatcher ousted Labour leader James Callaghan by just one vote.
If that happens, the prime minister usually calls a general election, with voters heading to the polls to vote for their MPs again. The party that emerges victorious in that vote then gets to form a new government, typically with a new leader at the head.
Whether leadership or general, elections can take weeks to organize and decide. And that's time the U.K. can ill afford to waste, given that Brexit is mere months away and the country still hasn't signed off on its deal to leave the EU.
Additional reporting by Newsy affiliate CNN.