The best news yesterday for Boris Johnson was written by Tony Blair’s former Director of Communications. Alastair Campbell brought out a lengthy denunciation of Jeremy Corbyn under the headline “Why I no longer want to be readmitted to Labour”, and repeated this message in numerous broadcasts.

There could be no more striking proof that the Opposition is for the time being irreparably split. Labour Remainers feel so betrayed by Corbyn that many of them have decided to switch to the Liberal Democrats.

It is true that Tory Leavers felt so betrayed by Theresa May that at the recent European elections, they switched in huge numbers to the Brexit Party.

But the Conservative Party reacted to that shock by electing a new Leader, and the Brexit Party is now being squeezed, as Mark Wallace noted on Sunday in his analysis of the most recent opinion polls.

The nearer the Johnson Government comes to delivering Brexit, the tighter that squeeze will become.

But suppose Parliament manages to block Brexit? Can Tory Remainers, including such resolute figures as Dominic Grieve, find some way at the eleventh hour, in alliance with the Opposition parties and with intransigeant negotiators in Brussels, Dublin and other European capitals, to delay the whole process, or even to bring it to a juddering halt?

This is a question on which it is possible for well-informed people to disagree. We find ourselves bombarded with mutually incompatible assertions about what can and cannot happen.

Johnson, however, is building a fallback position. If as he takes the country towards No Deal he finds his path blocked, or severely impeded, by a parliamentary coalition of Remainers, he can say, “Very well, let us have a general election, and let the people decide whether they wish to proceed with Brexit.”

What then will Labour MPs decide to do? Most of them know, as Campbell has pointed out, their party is in no condition to win such an election, and could indeed be destroyed by it.

In a Brexit election, Labour cannot be as welcoming to Remain voters as the Liberal Democrats, or to Leave voters as the Conservatives.

Jo Swinson has yet to make an impact as Lib Dem leader, but she is at least much newer than Corbyn, and much clearer on the European issue.

And Johnson is not just clear on Europe. By hastening to announce the increased spending on various public services which one would expect to find at the heart of a Labour manifesto, he has stolen the clothes a more dynamic Labour leader would already be wearing.

Many Labour MPs probably do not know themselves what they would do if confronted by the threat of an early general election. The problem for their party is that they might split several different ways.

Some might reckon that under Corbyn, a leader whose abilities as a campaigner were underestimated in 2017, the party could still perform well.

Some might call for a new leader, which would be difficult or impossible to arrange in a hurry, or for a Lib-Lab pact, which again would be difficult or impossible to arrange in a hurry.

And some might decide to do just about anything to avert an early general election. They might decide to accept Johnson’s argument that proceeding at full speed towards No Deal is the only way to obtain the necessary concessions from the EU.

Divided parties seldom do well in elections. Nor do divided oppositions. One of the conditions for Margaret Thatcher’s success in the 1980s was the splintering of the Left, with the formation in March 1981 of the SDP.

Johnson is an ebullient campaigner, who plainly hopes that by mobilising public opinion, he can place pressure on MPs to support what the Government is doing.

Campbell writes of “Johnson unspeakably now prime minister and changing the dynamic of the political debate”. Those words indicate a certain haplessness, an inability to work out how to get to grips with this new opponent.

Johnson has “unspeakably” seized the initiative, and no considerable figure on the Opposition side yet seems to have the faintest idea how to deal with him.