LONDON — When the House of Commons returns after the strange December election, Britain will be a very different place. No matter the outcome, one thing is already certain: The country’s political center will have all but disappeared.

The leaders of the two main parties have made crystal clear that the concept of the political party as a broad church no longer holds any attraction for them. Accordingly, many of their more moderate colleagues have taken the hint and bowed out gracefully. One in 10 MPs has already decided to retire — a number to be supplemented by the dozens who will inevitably lose their seats on polling night.

With a few obvious exceptions — Ken Clarke, Geoffrey Robinson — these MPs are not party grandees hanging up their spurs after a lifetime of witty repartee across the Commons chamber. Largely, they are men and women who won elections between 1997 and 2010 under the premierships of Tony Blair, Gordon Brown and David Cameron.

Whatever their political differences, these three former leaders were convinced that the way to win elections was to attract as many voters as possible. So they reached for the center — the Third Way — and poured their efforts into wooing those who had never before considered voting for politicians of a different stripe.

Blair changed the very name of his party, turning old Labour New and replacing its totemic red flag with a rose to prove that, as his spin chief Peter Mandelson declared, New Labour was “intensely relaxed about people getting filthy rich.”

The leaders of the two main parties in Britain have made crystal clear that the concept of the political party as a broad church no longer holds any attraction for them.

Remember David Cameron’s mission to detoxify the Conservative Party by hugging huskies and building villages in Rwanda? It was all about saying to Labour voters: It’s OK to switch to the Tories.

None of this was all that long ago, but it feels like it. Since the 2016 referendum, “triangulation” has become a dirty word; compromise is now a sin. The electorate that Blair, Brown, Cameron and George Osborne courted so assiduously — the mythical Middle England voters they scrapped over so energetically in the 1990s and 2000s — have disappeared from our leaders’ hearts.

They have been replaced by an intense focus on Workington Man — the apocryphal Labour-voting Leave supporter that the Conservatives are said to need to woo in order to make up for all the disappointed Remainer Tory voters in leafy, wealthy places such as Guilford. (“Well, Guildford will have to go,” Johnson is said to have muttered when his MPs warned him the Surrey town would not tolerate a harsh Brexit.)

This election, it seems, is solely about Workington Man’s dilemma of whether to stick with Labour, reward the Tories for Prime Minister Boris Johnson’s Brexit deal or go whole hog and vote for the Brexit Party (if possible). While some have noticed that moderate MPs are fleeing Parliament, there’s been virtually no discussion of the voters who put them there — the moderates whose views are suddenly unfashionable.

Today’s leaders may not prize their vote. But this group of millions of centrist voters — many of whom are Remainers — still holds the key to No. 10 Downing Street in their moderate grasp. What they decide to do come December, in an election in which both Johnson and his rival in the Labour Party, Jeremy Corbyn, have repudiated them, will almost certainly be key to its outcome.

Take a former Labour voter, disgusted at Corbyn’s approach to anti-Semitism, uneasy at Labour’s radical economic policies and fed up with its lack of clarity over Brexit. Or a Tory, furious that the MPs they admire have been driven out, unconvinced by Johnson’s clownish ways and worried about the fate of their business in a post-Brexit world. What are they going to do on December 12?

One obvious choice would be to follow the lead of moderate MPs such as Luciana Berger and Sarah Wollaston by defecting to the Liberal Democrats. Yet the Lib Dems have already begun to slump in the polls, their pledge to cancel Brexit making many voters — even the Remainers — uncomfortable.

For most middle-of-the-road voters the Lib Dems aren’t even really an option. There are only a handful of Conservative seats where a 5 percent swing to the Lib Dems would oust the sitting Tory MP in favor of a Lib Dem candidate. But dozens where the same shift would deliver the constituency to Labour.

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That puts party loyalists in an awkward position. If you were traditionally a Labour supporter looking at the numbers in your home town, would you risk voting for the Lib Dems and returning a Conservative MP to the Commons? Or, as a Tory voter, would you really want to help deliver your bogeyman Corbyn into Downing Street via a minority government propped up by a rejuvenated Liberal Democrat Party?

Barring an unlikely Lib Dem surge in the next few weeks, it seems the moderates have two choices: to hold their noses and vote for the party they always have, or to sit this one out.

Neither option is likely to deliver the outcome they’d want to see: a return to the middle ground. And yet what they do with that choice will decide the future of Britain — and Europe — for decades to come.

Rosa Prince is the author of “Theresa May, the Enigmatic Prime Minister” (Biteback Publishing, 2017) and “Comrade Corbyn” (Biteback Publishing, 2016).

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