Do you mean like Bill and Hillary Clinton did in 1992 ?
Useless and phony; he would be neutered as a candidate. He could simply be honest. The truth is his best defense.
If it is now obvious he has lost so much support that he will lose next month, he could announce that he is resigning immediately as the Republican nomination in favor of Mike Pence. He could even let Pence take the next 2 debates. That would be a political earthquake.
Right now it looks like Congress, both houses, will remain Republican. If Trump is forced out or chooses to leave, his supporters will likely take Election Day off. The at the least the Dems take the Senate. No, Trump has to stay in .