LONDON — Just minutes after Prime Minister Theresa May’s plan for leaving the European Union was resoundingly defeated on Tuesday night, the leader of the opposition Labour Party, Jeremy Corbyn, rose in Parliament and vowed in thunderous tones to unseat her.

For Mr. Corbyn, a soft-spoken outsider who stormed to the top of the opposition party three years ago, the moment marked a crossroads. After years of lying low on the question of Brexit, he finally made his move, thrusting himself into the center of the debate. He demanded a no-confidence vote, considered the first step in having to choose a side in the Brexit debate.

In the end, Mrs. May clung to power as lawmakers beat back the challenge against her. But Mr. Corbyn has still emerged with an enormous amount of influence to shape the Brexit battle moving forward.

Not only did Mrs. May barely survive the no-confidence vote in the House of Commons, she also suffered the worst parliamentary defeat in Britain since the 19th century the day before, when her plan for carrying out Brexit failed 432 to 202, with more than 100 of her own party members deserting her.