Both Chaucer and Shakespeare wrote of the medlar fruit, which comes, appropriately enough, from the medlar tree. The special distinction of this fruit, and the reason for its appearance in literature, is that it goes rotten before becoming ripe. It is, in short, an excellent metaphor for Marco Rubio.

After his humiliating fourth place finish in Mississippi (with the no-longer-running Ben Carson not far behind him), and another fourth-place finish in Michigan, Rubio’s campaign is essentially finished. This was pretty clear after his dismal showing on Super Tuesday, and it was hammered home by his abysmal debate performance last week. Rubio’s consistently poor showing has been so surprising to so many political pundits (myself included) that his campaign will likely be remembered as one of the most disappointing of the past several election cycles. After Tuesday night’s big losses, Rubio is left with very few options. But he does have one final opportunity to help stop Donald Trump: drop out immediately.

Trump once again had a very good night on Tuesday, despite recent headlines about declining poll numbers and a lower ceiling of support. And Ted Cruz’s second-place showing in Mississippi and essentially tied-for-second-place finish in Michigan entrenched the sense that he is the anti-Trump candidate. (One interesting exit poll finding, from Michigan, showed Cruz beating Trump 44-39 in a two-person race.) Yes, Kasich was bunched up with Cruz in Michigan, and might win his home state of Ohio next week. But after that he will be short on money and support. He has shown himself incapable of winning anywhere to date, and for most of the campaign he has been so irrelevant that his rivals have not even bothered to attack him.

Which is why Rubio should do everything he can to boost Cruz, the only non-Trump viable candidate, immediately. This means dropping out before his home state of Florida’s March 15 primary, even if he could win it. Recent polls in Florida have shown Rubio in second place, but his support nationwide is falling like a rock. There was a non-Trump case to be made, a week or so ago, that Rubio should stay in to deny Trump Florida’s winner-take-all delegates, despite it meaning a more crowded field. But the most recent poll of Florida has Cruz five points behind Rubio; given the different trajectories of their campaigns, and Rubio’s further anti-momentum from Tuesday night, it seems reasonable to conclude that a Cruz victory without Rubio in the race may be as likely as a Rubio victory with all three of them still running. If Rubio wins Florida, that’s just one victory for him. If Cruz can win Florida, he’ll add to his not-insignificant delegate total.

Admittedly, a Cruz win in Florida is not a very likely scenario, in part because many early voters have already cast their ballots for Rubio or Trump. But a Rubio exit would allow Cruz to pick up at least some of Rubio’s support nationally, which he has already been doing all week. Upcoming states like Missouri and North Carolina are much more likely to go to Trump in a crowded field; if Cruz can pick up many of Rubio’s votes in these states, he could win either of them. Rubio’s immediate departure would also give Cruz a more direct shot at Trump at Thursday’s debate and over the next week on the campaign trail. Given how poorly Trump performed in the last debate when he faced sustained attack, this could be the only strategy, however tenuous and unlikely, to take him down.

Rubio spent the past 10 days of the campaign saying that Trump could not be the nominee of the party; in all likelihood, Trump is going to be the nominee no matter what Rubio does. But there is still a tiny chance to prevent that outcome, and it may hinge on Rubio getting out of the race. If he does so, Rubio would prove that his vacuous and disappointing campaign had at least one small bit of purpose to it.

Read more Slate coverage of the GOP primary.