P HILIP SAGAR is a Conservative from central casting. The chairman of Grantham and Stamford Conservative Association has a clipped moustache, two manager-of-the-year awards as a hotelier, and a gripe with the local MP. “We’ve got another MP who has gone rogue on us,” he complained in a recent interview. The MP in question is Nick Boles, a liberal Tory who has tabled amendments to rule out a no-deal Brexit. This has not gone down well among party activists in a constituency where 60% of voters backed Leave. The upshot? Mr Boles finds himself in a fight over whether he can stand as their MP in the next election.

He is not alone. MP s from both the Conservative and Labour parties are wrestling with rumbustious activists threatening deselection. Conservative critics of Brexit, such as Anna Soubry and Sarah Wollaston, face similar rumblings to Mr Boles. Labour members are unhappier still. Between half-a-dozen and 30 Labour MP s are at risk of deselection, suggests one shadow minister. Luciana Berger, a Liverpool MP , faced a motion of no confidence from local activists after criticising the Labour leadership and its handling of anti-Semitism in the party. The motion was pulled on February 8th after it was pointed out that pursuing one of the party’s most prominent Jewish MP s—heavily pregnant to boot—would hardly reassure those who worry about Labour’s streak of racism.

Deselection is not straightforward in either party. Tory MP s must lose two rounds of votes, first among senior local party officials and then among the local membership, before they lose their candidacy. In the Labour Party, a third of constituency branches or affiliated unions can force a sitting MP to undergo a competitive reselection, rather than automatically standing again. In both cases the process involves volunteers spending long evenings arguing with each other.

But a new breed of party member is increasingly happy to jump through these hoops. There are a few reasons why. The first is that, in both main parties, since 2015 rosette-wielding door-knockers have had the final say on who becomes leader. In the past, MP s were reselected before every general election with few questions asked. It has become easier for members to keep tabs on their MP ’s voting record online. And now that members pick the leader, it seems odd simply to nod through any old MP without comment.

A form of populism, in which the virtuous people rise up against a corrupt elite, has got into both parties, says Tim Bale of Queen Mary University of London. The virtuous are the grassroots members. In the Conservatives, the role of the corrupt elite is played by those MP s seen to be betraying Brexit. In the Labour Party, it is those MP s who criticise the leader, Jeremy Corbyn. Hard-core Brexiteers in the Tories and Mr Corbyn’s keenest supporters in Labour merrily stoke these feelings.

Ideological impurity is not always career-threatening. In Rushcliffe, a 20-minute drive from Mr Boles’s Grantham seat, another outspoken Tory Remainer, the avuncular Ken Clarke, has not faced calls to step down. When members take against their MP there has usually been a personal falling out or a feeling that the MP is not pulling their weight. (Mr Sagar suggested in a newspaper column that some members in Grantham considered Mr Boles “out of touch, arrogant, remote and living in the Westminster bubble”.)

Those higher up the party food chain can step in if members get out of hand. Conservative HQ has leant on local associations to back its preferred candidates, most notably under David Cameron in 2005-16. An attempt to deselect Mr Corbyn when he was a rebellious backbencher was supposedly kiboshed at Tony Blair’s behest. And the unions have reason to keep the power of Labour members in check, lest their own ability to parachute candidates into seats is weakened.

Still, the threat of deselection casts a long shadow. Many Labour critics of Mr Corbyn or Tory critics of Brexit keep their heads down. Mr Sagar and his fellow members are on the march.