For the House of Lords, the unelected counterpart to the House of Commons, Wednesday could mark a rare moment in Britain’s politics: The ermine-robed barons and baronesses of that ancient chamber will vote on whether to defy an elected British prime minister over a flagship policy.

The Lords are scheduled to hold a pivotal debate on the policy, which would put asylum seekers on one-way flights to Rwanda. They have attached multiple amendments to the bill in an attempt to water it down; the government, with its hefty Conservative majority in the Commons, has systematically stripped them off.

Nobody, least of all the Lords themselves, believes that the upper chamber will ultimately torpedo the legislation. In the unequal clash between the elected Commons and the unelected Lords, the Lords invariably yield. But they could delay its passage by another week or two, which could be enough to jeopardize Prime Minister Rishi Sunak’s goal of putting the first flight to Rwanda in the air by the end of May.

That would thrust the House of Lords into election-year politics in Britain in a way that is unusual for an institution that views itself as a coolheaded, deliberative check on the more unruly Commons.

The prospect of sending asylum seekers to an East African country — and overruling a Supreme Court judgment — has generated so much opposition, even from Conservative peers, that it has shaken the Lords out of their customary deference.

“This is about people having a fundamental objection to a piece of government legislation,” said Simon McDonald, a former head of the British diplomatic service who became a cross-bench, or nonpartisan, member of the House of Lords, where he is known as Baron McDonald of Salford, in 2021.