The Conservatives need Labour votes – that’s to say, support from some of those who backed Jeremy Corbyn’s party last year.  It needs them in much the same way that it needs Liberal Democrat, Green and UKIP votes, not to mention the backing of those that don’t vote at all.  But there is an important difference between Labour and the other political parties.  It took nearly 13 million votes, more than any other rival political organisation.  That’s a huge pool of potential supporters to fish in.  We were baffled by the exclusive focus of some during the Coalition years on UKIP, which never had more than two MPs at any one time, when Labour after 2010 had over 250.

It goes almost without saying that there will always be a limit to the number of red voters that a blue party can attract.  But it isn’t mission impossible to win a significant slice.  At the last election, the Party picked up Mansfield, Middlesbrough South and East Cleveland, North East Derbyshire, Stoke-on-Trent South, and Walsall North, former Labour seats all.  The local government election results of the previous month suggest that Theresa May would have won even more Labour-held constituencies were it not for her disastrous election campaign. The Conservatives missed out in Bishop Auckland by 502 votes, in Ashfield by 44, and in Newcastle-under-Lyme by only 30.  As our columnist James Frayne argues regularly on this site, in his “Far from Notting Hill” column, the road to a future Tory majority runs through midlands and northern marginals.  Margaret Thatcher wouldn’t have won three-figure majorities in 1983 and 1987 without the backing of former Labour voters who had used the Right to Buy scheme to purchase the council homes they lived in.

So we welcome the Prime Minister’s raid behind Labour lines in yesterday’s Observer.  And if it spins Momentum into a tizzy, so much the better.  But there seems to be more at play in her gambit than an appeal to the party’s voters.  So having agreed that it’s essential to make a pitch to Labour voters, let’s put aside for the moment the policy questions involved, with one exception.

Namely (and inevitably), Brexit.  May is looking for votes from Labour MPs to help get any deal she reaches through the Commons.  There a history to this particular quest.  There was a kerfuffle last July when it emerged that Gavin Barwell was planning to brief Labour MPs.  The suspicion among Brexiteer Tories was that the Prime Minister was preparing to push through a softer Brexit with their support.

Who might these MPs be?  The answer looks problematic for Downing Street.  First, one must assume that Corbyn and his front bench will vote against anything put up by the effing Tories.  Rule Number One of opposition is to do anything one can to undermine the Government, and dynamiting May’s Brexit plan would be a spectacular demonstration of the principle.  Then there are the pro-EU Blairites, together with a sprinkling of Brownites.  Yes, they might back the Prime Minister rather than risk No Deal.  But no, they might not – because our friends at the second referendum campaign would urge them to vote May down, so that Parliament could then manoeuvre to get No Brexit rather than No Deal: in other words, postpone moving Article 50.  Finally, there is the threat of deselection.  What Labour MP with a sense of self-preservation would vote with the Tories?  The Prime Minister’s best chance is with anti-Corbyn Labour MPs who dread No Deal, feel a sense of duty to the nation – and plan to retire before the next election.  Up yours, Momentum!

But the more blatant her pitch is to Labour MPs, the more she risks riling Conservative ones.  Number Ten is playing a dangerous game.  None the less, it is one with strategic purpose.  Never forget that a tranche of Labour MPs believe that Corbyn is unfit to become Prime Minister.  If they could be peeled off into some sort of National Labour or Democratic Labour pool in Parliament, then May could do what Thatcher did after the formation of the SDP: divide and rule – gaining the elusive big majority that has eluded the Conservatives since, well, those very Thatcher years.  The Brexiteering Right could thus be marginalised.  Some in Downing Street would welcome that.

Never under-estimate, though, the cultural resistance of Labour MPs to the Conservative Party.  They know their recent history, too: memories of how the Left divided during the 1980s are very raw.  And there is another wrinkle.  It’s no use splitting a rival party if one simulaneously splits ones’s own.