Alastair Campbell has a plan to aid tactical voting – but will it work?

The former New Labour spin doctor has drafted two letters: one to Labour supporters, stressing that he's been Labour all his life and thinks they should vote Lib Dem in certain marginals.

But he also written another, which stresses that he's an editor at anti-Brexit paper The New European, has impeccable Remain credentials, and thinks they should vote Labour in certain marginals.

Campbell is able to do this in part because he's been expelled from Labour - members can't endorse candidates from other parties. He says either party is welcome to use the letter in marginal seats where they want as long as they inform him.

It's an interesting idea and he's clearly trying to do his bit and using his profile to help oust the government.

The limitations of the plan probably the same as the limitations of any tactical voting, however. One question is whether the prospect of a referendum or stopping Brexit is actually going to be high on voters' agenda. Polling and focus groups suggest that Brexit has slipped down the rankings a bit as the decider of votes, eclipsed by issues like the NHS.

But even if Brexit isn't dominating, the letters don't have to appeal to all voters to make a difference. Campbell's interventions could help convince a rump of voters, however small, to vote tactically. That is what makes the difference in a number of seats.

What remains to be seen is whether the parties decide to use the letters.