Jay Cost doesn’t like  that Jeb Bush is staying in the race:
That is what makes the play so objectionable: the Bush campaign is trying to set up a race against Trump that the Bush campaign has no good reason to think Bush can win. And the odds are that all the Bush campaign will achieve is making more likely the calamitous nomination of Donald Trump. Yet Bush is determined to try to force a final showdown between him and Trump by dumping $20 million (so far) in negative ads against manifestly superior candidates.
It’s true that Bush’s determination to stay in the race (and to take Rubio down in the process) makes it easier for Trump, but frankly the same could be said of any of the other “establishment” candidates. They are all conspiring in their own defeat, and any one of them is just as responsible for that as the others. Bush’s favorability numbers are poor, but then so are Kasich’s and Christie’s, so if they were primarily interested in stopping Trump all of them should have dropped out months ago. Kasich and Christie have poor or non-existent campaign organizations, and Rubio’s is not that much better, so Bush could reasonably think that he has the best chance to scale up his campaign to compete across multiple states in a way that the others can’t.
The trouble is that all of them wrongly believe  they have the luxury of time, and they are encouraged in this by pundits that keep insisting that the race will be a long, drawn-out affair. Never mind that many of the pundits saying this are obviously in Rubio’s corner . The other “establishment” candidates hear these arguments and seem to believe them as well. Each of them looks around at his rivals and thinks, “Why not me?” The persistence of the Rubio fantasy gives them encouragement that one of the other “establishment” candidates might play the role that Rubio is supposed to fill.
If the delegate race is going to be the long slog that Rubio needs it to be, Bush has just as much incentive to delay his withdrawal from the race as Rubio does. Maybe Rubio has a better chance than Bush, but at the moment the senator’s “manifest superiority” is not manifesting itself. If it had, he probably wouldn’t be trailing Kasich in New Hampshire  and barely hanging on to third place in South Carolina . Looking ahead to Florida  doesn’t help the argument for Rubio or Bush, since both of them trail Trump and Cruz badly in their own state.
Like the dynastic candidate he is, Bush believes he was entitled to deference from his fellow Floridian, and he and his allies resent Rubio for having the audacity to get into the race. Presumably they blame Rubio for taking away support they think Bush would have had, and they see him as an ingrate who needs to be taught a lesson. They are also probably convinced by the argument that “establishment” candidates will fare better in states that vote later in the spring, so they don’t think there is any need for them to abandon the race. Bush and his allies are still working off of a theory of the race that is already being proved wrong right now, but they have committed themselves to it this far and they probably aren’t going to give up before Rubio does.