An oft-repeated claim in the media is that the party went easy on Donald Trump in terms of attack ads in the early contests and until very recently. Based on SuperPAC spending data from the FEC, this appears to be false.
Except in New Hampshire, Trump was the #1 target of SuperPAC attack ads. In New Hampshire, the biggest target was Chris Christie. The vast majority of that anti-Christie spending in New Hampshire was by the pro-Rubio super PAC, which may be part of the reason why Christie was particularly angry at Rubio and decided to go with Trump to dispel with the fiction that Rubio has a chance to win.
Rubio is actually the one who's had it relatively easy in terms of attack ads, and he's had tons of money spent on his behalf in pro-Rubio ads. Trump has had very little SuperPAC spending (I haven't managed to compute it out yet). Yet Trump's been winning.