Noah Millman reviews  Rubio’s declining performance and concludes that Rubio’s continued presence in the race helps Trump. He adds that party leaders could try to push him out, but they don’t want to:
But the GOP establishment doesn’t really want to stop Trump. What they want is to back the candidate of their choosing, someone they know will be reliable on the issues that matter most to them, and who they also believe they can sell. They have made excuses at every twist and turn of this campaign in terms of how different things would have been if just one little thing had gone differently – and bewailed the good electoral fortune showered on candidates whom they cannot abide. But they cannot reconcile themselves to the fact that it’s not bad luck. The primary electorate really, really doesn’t want to vote for someone the party leadership has blessed. Frankly, they’d prefer someone completely unacceptable to that leadership.
Rubio likes to insist that he isn’t an “establishment” candidate, because even he understands at some level that Republican voters don’t want a candidate that party leaders and donors prefer, but he was perceived that way because that is how he has acted and presented himself to voters. He has tried to sell himself as the most electable candidate, and he talks up his ability to “unite” the party, and these are both traits that Republicans are used to associating with the “establishment” candidate. He sticks to the party line on virtually everything, and on immigration voters assume that his real loyalty is to what serves the interests of the wealthy. Rubio’s trouble is that he was stuck with the baggage of the party “establishment” without gaining any of the traditional advantages, most of which had already been taken by Bush, and he did this in a year when those advantages counted for less than ever before. On top of all that, by spoiling Bush’s chances he earned the enduring resentment of Bush and his allies without getting any credit from the voters that don’t like them.
Meanwhile, Rubio keeps pretending  that his campaign hasn’t failed:
He also said the states that have yet to vote will be more favorable for him going forward. “This map only gets better for us as we move forward [bold mine-DL] in some of the other states,” he said. “We’re soon gonna be in the winner-take-all process…that’s where we feel very confident as we move forward.”
Some of this is the nonsense that losing candidates have to say as their campaigns wind down, but some of it reflects the serious errors in Rubio’s campaign strategy from the beginning. Everyone else looks at Rubio’s serial losing and correctly declares it a failure, but according to Rubio’s campaign this is mostly what they expected to happen. They claim to believe for some reason that as the contest moves into winner-take-all states in the Midwest and Northeast that the map “gets better” for them, but this is clearly not true.
Just as many observers failed to anticipate Trump’s strength in blue-state primaries, Rubio boosters have mistakenly expected him to do well in blue states that are probably even less hospitable to him than the states that have already voted. Rubio is an “establishment” candidate who has never dominated among moderate and “somewhat” conservative voters as an “establishment” candidate normally does, so he will likely do worse in almost all of the elections coming up in the next week and a half because the map is actually getting worse for his campaign. Rubio’s campaign relied on the belief that it was possible to win by losing dozens of races before somehow reviving suddenly to seize the biggest prizes, but there was never any good reason to think this would work. The idea that Rubio would succeed in his long-shot bid for the nomination was always a fantasy, and it tells us everything we need to know about the political judgment of the senator and his many boosters that they have believed it for this long.