If the Republican party schism breaks wide open at their convention

[reflection]

and the Democrats retain the Presidency, retake the Senate, and gain seats in the house (I know, a lot of ifs, but very possible) how many moderate (I know, relative term) Republicans will try to change their party affiliation in order to get with the side that will currently have the most influence? I really don't want to see any more influx of Republican ideology into the party, but bolstering the numbers are attractive.

And a serious question, because I honestly don't know, please don't flame, but I assume if a particularly odious Republican (e.g.: Cruz, just using him as the worst example, he'd never do it) wanted to cross over, there is a mechanism in place by which the Democrats could flat out reject him, right?