LONDON — British Prime Minister Boris Johnson’s chances of success in next month’s election got a boost Monday after Brexit Party leader Nigel Farage offered a truce.

Farage said he would not field candidates for the 317 seats that Johnson’s Conservative Party won in 2017. That means, in half of Britain, voters who hunger to leave the European Union will not have to decide between a pro-Brexit Conservative candidate and a really, really pro-Brexit Brexit Party candidate.

This represents serious reprieve for Johnson, whose campaign launch last week for the Dec. 12 election was marred by comments from prominent party figures who insulted the victims of a notorious fire and played into criticism that the party is out of touch.

Conservative Party leaders had feared that if Farage made good on his earlier threat to fight for all 650 seats in Parliament, votes could be split between them, giving an advantage to the opposition Labour Party, which takes a fuzzy stance on Brexit, and the Liberal Democrats, who oppose leaving the European Union.

But Farage backed down from his threat on Monday and said his focus will be on beating Labour and stopping those who support a second referendum on Brexit, a position endorsed by Labour, the Liberal Democrats and other minority parties.

“What we will do is concentrate our total effort into all of the seats that are held by the Labour Party, who have completely broken their manifesto in 2017,” which supported Brexit, he said.

Labour has had its own stumbles beginning its campaign.

First, Deputy Leader Tom Watson abruptly resigned. Then four ex-Labour lawmakers urged voters to back Johnson and the Conservatives to prevent Labour leader Jeremy Corbyn from becoming prime minister.

Analysts spent Monday adjusting their electoral calculations.

John Curtice, a politics professor at the University of Strathclyde — who is considered a savant after anticipating the Brexit referendum outcome — said that, on the whole, “this is good news for Boris, but it’s not as good as it looks.” He explained that “Farage is not standing down in seats that Boris needs to gain from Labour.”

Johnson must top more than 40 percent of the vote to form a majority government — and avoid the perils of a hung Parliament or having to cast his lot again with the one-issue unionists in Northern Ireland.

The Conservative Party is seeking to win over Labour voters who do not necessarily like Johnson or the Tories but who do want to “get Brexit done,” as the prime minister is urging.

These “Labour Leavers” have been cast as white, mostly male, working-class rugby fans living in the faded industrial towns of the Midlands and North England, who usually support Labour but do not like Corbyn or his party’s stance on Brexit.

Elections in Britain usually are not won or lost by the brief campaign season. But there are exceptions. In the 2017 general election, the Conservatives lost a 20-point lead to Corbyn and his youthful “Momentum” movement.

In 2017, too, 80 percent of voters flocked to the two main parties, the Conservatives and Labour. But this election looks much more fragmented, meaning the two main parties are vulnerable on various flanks.

Last week, three of the smaller parties — the Liberal Democrats, Plaid Cymru and the Greens — agreed to a pro-remain alliance, saying they would step aside for one another in 60 seats.

On the Brexit side of the ledger, the truce with Johnson may not be the formal alliance that Farage wanted, but it is a pact of sorts.

“We now have a leave alliance; it’s just that we’ve just done it unilaterally,” Farage said Monday. He said that his announcement “prevents a second referendum from happening, and that, to me, I think right now is the single most important thing in our country.”

Johnson said in a statement, “We welcome Nigel Farage’s recognition that another gridlocked hung Parliament is the greatest threat to getting Brexit done.”

Jo Swinson, leader of the pro-E.U. Liberal Democrats, tweeted, “The Conservative Party are the Brexit Party now.”