LONDON — Prime Minister Theresa May had just suffered three humiliating defeats in Parliament over plans to leave the European Union and, with a critical vote less than a week away, Britain seemed to be facing its biggest political crisis in decades.

So the stage was set for the opposition leader, Jeremy Corbyn, to deliver a knockout blow.

But when he rose to address a rowdy Parliament chamber on Wednesday, Mr. Corbyn, the left-wing leader of the Labour Party, ignored the chaos over Europe, instead using his allotted questions to focus on poverty and welfare changes.

Throughout the interminable debates over Britain’s withdrawal from the European Union, or Brexit, Mr. Corbyn has often attacked Mrs. May, but he has been conspicuously noncommital about what he would do instead.

To this point, Mr. Corbyn’s strategy has served him well.

Yet with Parliament expected to reject Mrs. May’s Brexit plan on Tuesday, the space for dodging uncomfortable issues is rapidly dwindling, and a growing question is where Mr. Corbyn will come down. Whatever his decision, it could have a fateful impact on the result.