LONDON — Prime Minister Boris Johnson’s bid for a pre-Christmas election got a major boost Tuesday when the main opposition Labour Party said it would back a December ballot, almost guaranteeing an early election that will be dominated by Brexit, Brexit and more Brexit.

Jeremy Corbyn, leader of the Labour Party, said a decision by European leaders on Monday to delay Brexit until Jan. 31 met his condition that, at least for the next three months, the threat of a no-deal Brexit is off the table and Britain could not suddenly “crash” out of the trading bloc.

“Labour backs a general election, because we want this country to be rid of this reckless and destructive Conservative government...,” Corbyn said in the House of Commons on Tuesday. “We will launch the most ambitious, radical campaign for real change.”

If elections come in December, voters will be offered some stark choices over Brexit — alongside the usual muddle, misrepresentations and foggy numbers.

The Conservative Party under Johnson will run as the 100 percent for Brexit party, under the banner, “Let’s get it done.”

“There is only one way to get Brexit done in the face of this unrelenting parliamentary obstructionism, this endless willful fingers crossed ‘not me, guv’ refusal to deliver on the mandate of the people,” Johnson said on Tuesday, “and that is to refresh this parliament.”

The new Brexit Party, led by talk show host Nigel Farage, will be 200 percent for Brexit, arguing: Why even talk to the Europeans anymore? Let’s crash out.

The Liberal Democrats will campaign to stop Brexit. And have a second referendum.

The Scottish National Party will push to remain in the European Union — and, by the way, promote Scottish independence.

And then there’s Corbyn’s Labour. That one is a puzzler. The center-left party’s manifesto will be: Let’s win the election; then let’s negotiate a very soft, very closely aligned “Labour Brexit” with Europe; then let’s have a Labour Party conference to decide whether to support the deal; and finally let’s have a second referendum to see if the people support it.

There is still some question about the exact date of the snap election.

Johnson’s government on Tuesday published a short bill calling for an election on Dec. 12 — but it might agree to a slightly earlier date, to win over opposition parties.

There has been much haggling over the timing in the past 24 hours. But the fact that Labour, which previously resisted a snap election, has in principle given its backing means that it’s now highly likely that there will be a general election in December.

The government’s election bill is set to be debated in Parliament on Tuesday, followed by a vote on Tuesday night.

The bill could still be amended. On Tuesday afternoon, Labour’s Stella Creasy won her bid to allow amendments from any lawmaker.

Richard Burgon, also from Labour, said his party would push to allow E.U. citizens living in Britain and British 16- and 17-year-olds to vote. Both groups tend to support maintaining a close relationship with the E.U. — and oppose the hard split Johnson envisions.

But even if such changes aren’t approved, Burgon said, “when push comes to shove, we’re going to support a general election because people in our country deserve better than Boris Johnson and the Tories.”

“They believe they are born to rule,” he said. “They are going to now find out that they are not.”

The momentum toward an election is a bit of rare good news for Johnson, who has suffered defeat after defeat since he was selected as the leader of the Conservative Party and prime minister in July.

Lawmakers approved in principle the Brexit deal he negotiated with E.U. leaders, but then the House of Commons said no to the fast-tracked timetable Johnson wanted to get the legislation approved.

In that vote, he needed the support of two-thirds of lawmakers — and fell far short. Tuesday’s bill requires only a simple majority.