Former President Donald Trump has struggled so far to land a compelling line of attack against Vice President Kamala Harris, and former Republican insider David Frum has a theory about why. Writing on X, Frum argues that some of the negative attacks Republicans have thrown at Harris and Minnesota Gov. Tim Walz would have had more of an impact if not for the fact that the alternative the GOP is offering is former President Donald Trump. “The problem Republicans have is that every election is a compare and contrast,” he writes. “And they overwhelmingly re-nominated a candidate who… entered pol…