Tory prime ministers need to keep their party members happy if they want to remain in power. Otherwise they’ll quickly get replaced by somebody who’s better at sooking up to them. (This is to some extent the case in all parties, but the way Conservative leaders get deposed and elected makes this even more true for them.) Of course it’s also important to win elections, but when they’ve been in power for a while, other things matter a lot, too.
The recent study of party members’ attitudes to various questions is therefore of great interest. Tory grassroots strongly believe that the UK should leave the Customs Union (CU) and the Internal Market (IM). They’re also adamant that there shouldn’t be a second Brexit referendum (probably because they know they’d lose it). They also overwhelmingly want the Home Office to treat EU citizens like any other foreigners, which will be a massive change from the status quo (and would be likely to make the EU reciprocate towards UK citizens on the continent).
Whereas most members of Labour, the Lib Dems and the SNP would be delighted if Theresa May decided to remain in both the Customs Union and the Internal Market (as would a majority of people in the UK, I expect), it’s clear that the Conservative party wouldn’t accept this. Although I don’t believe the survey asked about it, I reckon they hold similarly tough views on ECJ jurisdiction. In other words, Theresa May’s red lines were drawn because the Tory grassroots would have defenestrated her otherwise. As as we know, those red lines lead directly to a Canadian-style deal, as demonstrated by Barnier’s famous staircase:
Many people don’t believe Theresa May would implement a solution that would be disastrous for the UK, but if the survey is accurate, she’s effectively being held prisoner by her party. The Conservatives are so convinced that there’s a pot of gold of the other side of the iceberg that they’re deliberately sailing the Titanic into it. Unless somebody manages to trigger a new election before it’s too late, the Tories will continue to pursue a hard Brexit, no matter what.
I’d like to think the Tories will be utterly unelectable two years after Brexit, but that won’t bring back the jobs they are currently sacrificing on their xenophobic and imperialist altar.