The Democratic Party has become, to a significant extent, an anti-racist party. The Republican Party has not.So, which Democratic leaders have been forced to retire for having made racist remarks? Who's the Democratic Party's Trent Lott?
In an anti-racist party, politicians who demonize historically discriminated-against groups are either forced into retirement or, at the least, forced to apologize. Obviously, what constitutes bigotry is not always self-evident. But if many of the members of a historically discriminated-against group perceive something as bigoted, that’s usually a good hint.
Lott was the Senate Majority Leader. There's no way that a Democrat of similar standing will ever resign for making bigoted remarks, and it's not because they don't make them. I'm not going to reprint the remarks at those links, but if you need proof of the claim, follow the links.
The Republican Party has been effectively more anti-racist than the Democratic Party by the standard of actually forcing its leaders to resign over racist remarks. They may do this because they think racism hurts them more -- as David French says, they're sensitive to the charge that they're shot through with xenophobia. Being 'on the right side' gives Democrats cover to act out without consequences. Joe Biden or Harry Reid can say things that would cause a Republican to wither on the vine, and nothing happens to them at all.
The reason that party leaders are not endorsing Trump right now is precisely because they are appalled by him. Some of it may be tactical, but some of it is not. It's also tactical to try to claim that Democrats are the anti-bigotry party. The evidence does not support this conviction.