Guys, no one has run a serious ad campaign against Trump since he's gotten into the race because they're all scared to alienate his supporters.
Hillary will not hold back one bit and her campaign team will absolutely tear Trump to pieces over the air. Not to mention, Trump is going to have to spend a lot of political goodwill to earn the support of conservatives that loathe him almost as much as Hillary for his attacks on Cruz and his inconsistencies, as well as Bush style neoconservatives who consider him a national security threat. The Democratic race is much tamer, and Sanders will likely give his full throated endorsement to Hillary when he loses.
Trump might have a real shot if he can win the nomination by the end of March and if Sanders can drag out the race enough where it starts to get really nasty. He could consolidate the GOP in time in that situation, but if Cruz and Rubio can prevent Trump from winning it before the convention, it'll be a McGovern/Goldwater repeat.
Also it's not as though Republicans in Congress are exactly going to be rooting for Trump. The 2018 Senate map is very favorable to the GOP, so they have to be relishing running in a midterm year against Hillary, and then running in the 2020 Prez race to beat her. Since the Dems would have held the White House for 12 years by then, the odds are decent the GOP will win, especially if there is any kind of a recession before 2020 (which there's a good chance there will be).
The biggest threat to the GOP from a Trump candidacy is that he costs them their Congressional majorities.