“I think there are millions of us,” says David Gauke as he hands out election leaflets in Berkhamsted high street. “Politically homeless people who want reasonable and respectful politics back.” The former justice secretary and anti-Brexit rebel, who is running as an independent in South West Hertfordshire, says Tuesday’s TV debate between Boris Johnson and Jeremy Corbyn has simply made things worse. “Many, many voters this morning have been telling me they are disheartened. They want a better choice.”

In the Tory heartlands of the commuter belt, remainers may revolt | John Harris Read more

It is often said that UK general elections are won and lost in the English Midlands. But British elections are regional and multi-party now, and there are plenty of swing constituencies in the London commuter belt where disgruntled voters have seized their chance to reshape Britain’s government in the past – and may do so again this time.

Traditionally, when Labour wins a majority in parliament it wins seats in counties such as Hertfordshire, Essex and Kent. Tony Blair won five out of 11 in Hertfordshire in 1997. But in 2017, Labour did not win a single seat in the county. Only an exceptionally obdurate Labour optimist can expect Jeremy Corbyn to improve on that on 12 December.

Yet plenty of these commuter-belt seats are up for grabs in 2019. That’s especially true of the areas closer to London, where younger families have migrated from the metropolis over the years in search of better housing and a greener lifestyle while still commuting into the capital. Many also voted to remain in the EU. Among them were three seats that face important election contests: Beaconsfield, South West Hertfordshire and St Albans.

Labour has no chance of winning any of the three. Beaconsfield and South West Hertfordshire are in play because their former Tory MPs, Dominic Grieve and Gauke, thrown out of the parliamentary party by Boris Johnson for opposing a hard Brexit, are running as independents, while St Albans is a major target seat for the Liberal Democrats’ Daisy Cooper. Having spent time on the campaign trail in all three seats this week, I think it is clear that the official Tory candidates are genuinely under threat in all of them. But it is also clear that the Tories will not lose without determined and large-scale tactical voting.

Right now, there is no common pattern. Yet uncertainty about how to make tactical voting work is widespread. This partly reflects the lack of information, which several websites and campaigns are now seeking to correct. But it also reflects a deeper cultural tension between the centre left and centre right, in which it is genuinely hard to persuade adherents of one tradition to vote tactically for the other, even though the substantive differences between them are sometimes small. Even among the reasonable and respectful voters whom Gauke invokes, tribal loyalties can be tenacious.

Cooper knows this at first-hand better than most. The former campaigns director for Hacked Off was widely expected to take remain-backing St Albans for the Lib Dems in 2017 and to oust the Tory, Anne Main, a hardline leaver who narrowly survived a deselection attempt in 2009 after an expenses row. But Cooper fell short on 32%, largely because the Labour vote held up at 23%.

On the doorstep with Cooper this week as she canvassed undecideds, it was hard to believe she will fail a second time. “I’m going to vote Lib Dem because I’m a remainer,” said Caroline, a former Labour supporter. This was a constant refrain. Stopping Brexit was by far the most common topic on the doorstep. If the Tories are to lose St Albans next month, Labour voters are going to have to vote for Cooper in greater numbers than they did two years ago.

Round the M25 in the Buckinghamshire seat of Beaconsfield, Grieve’s challenge is to persuade enough of the nearly 37,000 people who supported him as a Tory in 2017 to back him as an independent. “Corbyn is the bogeyman,” Grieve told me. “There’s an anxiety about somehow letting Labour come through and win.”

Judging by his reception from commuters outside Gerrards Cross station on the coldest morning of the year so far, however, plenty have overcome any such anxiety. Support for Grieve’s anti-Brexit stand was overwhelming. “Thank you for your integrity,” “You’re a principled man,” and “I can finally vote for someone who puts the national interest first” were just some of the accolades he received in the frosty dawn. “After a session like that, I think we might just do it,” Grieve told me later, over breakfast in a local cafe. He surely can.

Grieve is helped by the fact that in Beaconsfield the Liberal Democrats have stood down and endorsed him. Up the road in South West Hertfordshire, Gauke is not so fortunate. His campaign has had to start from scratch. The word here is that the local Lib Dems were prepared to give Gauke a clear run but that Jo Swinson and the national leadership overruled them, partly on the grounds that they did not want to do too many deals with former Conservatives. In practice, though, there is no sign yet of a Lib Dem effort there, and Gauke gets a supportive reaction from shoppers in Berkhamsted. The former Labour MP for Barnsley East, Michael Dugher, who lives locally, is backing Gauke too.

These are just three Conservative-held seats. Yet they stand for many others. Voters in such seats who want to stop Johnson from getting a majority, or who yearn for Britain to remain in Europe, have to get real about their options. If Labour cannot win these seats – and at present it cannot – then it is right to vote for the opponent with the best chance. This is not a time for political piety, prissiness or pride. It’s about facing the Brexit danger – and taking the one real chance of preventing it.

• Martin Kettle is a Guardian columnist