The House of Commons in March rejected the idea of a no-deal Brexit by a handsome 43-vote margin.

Yet last week, Dominic Cummings, Boris Johnson's adviser, suggested that it was now too late for MPs to stop Britain leaving without a deal on Oct. 31, the latest Brexit deadline.

This position was echoed by a Downing Street spokesman and by the health secretary, Matt Hancock, who was previously strongly opposed to no-deal.

There are two parts to the argument. The first is that Oct. 31 is now the default option, legally binding on both Britain and the European Union. In the absence of some specific action, such as agreeing to another extension, Brexit will take place then. The second is that, given the imminence of the deadline, MPs do not have enough power or time to prevent no-deal — unless the government cooperates.

And Johnson will not do that. Downing Street is threatening to force a no-deal Brexit even if the prime minister loses a no-confidence vote.

Does he mean it? It would be sensible to take the latest bluster with a pinch of salt. Johnson has two clear bargaining reasons to talk up the risk of a no-deal Brexit on Oct. 31.

One is to ensure that Brussels takes the notion seriously, which it did not when Theresa May was prime minister. That should raise the pressure on the E.U. to drop its refusal to reopen the withdrawal agreement. The second is to win back voters from Nigel Farage's Brexit Party, who positively favor no-deal. The Tories' loss of the Brecon by-election on Aug. 1 confirmed that, even under Johnson, they are still vulnerable to Farage.

Even so, a clear majority of MPs still oppose no-deal. Opinion polls suggest most voters are against it as well. Although Johnson has required all his ministers to sign up to the possibility, several are known to have been fretful about the consequences, including Michael Gove, who is in charge of preparing for it.

About two dozen Tory rebels have indicated that they are ready to join any cross-party efforts to stop a no-deal Brexit. They include several of May's former Cabinet ministers, notably Philip Hammond, David Gauke, Greg Clark and David Lidington.

Yet there are big hurdles in the way of attempts to prevent a no-deal Brexit. Some look back to March, when MPs succeeded in hijacking the Commons agenda, which is usually controlled by the government, to pass an act requiring May to seek an extension of the Article 50 Brexit deadline.

But this was possible only with the peg of legislation or an amendable motion. Johnson's team say neither will be needed or allowed before Oct. 31.

Are there other routes? Chris White, a former adviser to Tory whips now at Newington Communications, reports talk in Westminster of using emergency debates or opposition days. Rebel MPs expect help from the speaker, John Bercow, who seems willing to tear up normal procedural rules if need be.

But the government will not allow any opposition days. The shortage of parliamentary time acts in its favor — fewer than 30 sitting days are planned before Oct. 31 (rebels are therefore seeking a way to cancel the autumn recess). And there is no majority for the drastic option of revoking the Article 50 Brexit letter.

Such uncertainties explain why many MPs now talk of a vote of no confidence. The Labour leader, Jeremy Corbyn, promises to propose one soon after the Commons returns on Sept. 3. Johnson's government has a working majority of just one, so it requires only a handful of Tories to switch sides for a vote to succeed.

Yet Johnson will say he needs more time to secure a deal. It is hard for backbenchers to vote down their own government, which may be why since 1945 only one vote of no confidence has succeeded, against Labour's James Callaghan in 1979.

The rules were also changed by the Fixed-term Parliaments Act of 2011. Previously, any vote of no confidence would trigger the prime minister's resignation and a general election. But the 2011 act allows a period of 14 days during which either the sitting prime minister or an alternative tries to form a government that can win MPs' confidence. Only if these attempts fail must an election be called, on a date fixed by the outgoing prime minister.

As Catherine Haddon of the Institute for Government, a think tank, notes, it is not even clear under the act that the prime minister must resign, though a refusal to do so would produce a constitutional row that might even involve the queen.

Some MPs hoping to block no-deal are discussing the formation of a cross-party "government of national unity" to replace Johnson's, with the express purpose of asking for another Brexit extension to allow time for an election. The idea would be that a veteran such as the Tories' Ken Clarke or Labour's Margaret Beckett might be its nominal leader. Yet it is hard to see this working. Labour is not keen, and is likely to insist instead that any alternative government must be led by Corbyn.