In the end, it doesn’t really matter. If Flake sinks the nominee, he’ll need EVERY Democrat to go with him, including the ones who are facing defeat in Red States even without this coming up. These are the Democrats who MUST appear as ‘moderate’ to win (even though they’re not, of course).
Bottom line - the Dems, if they can block the nominee, will lose 4 or 5 seats in November, and Trump will simply re-nominate him - and he sails through.
That is thinking two moves ahead. And that would be a good scenario.