Jeb Bush's campaign has been struggling in the polls for a while now, and it looks like fundraising has taken a big hit too because the campaign is downsizing and cutting salaries in order to reduce its burn rate. Still, Jeb has plenty of advantages. The most endorsement, the most money in the bank, the best name recognition, and a poll standing that still generally puts him ahead of the real politicians in the race. The campaign season hasn't gone nearly as well as he would have liked, but he's far from out of this thing.
But if he cares about his family legacy, the good of the Republican Party and the ideological principles he espouses, he should drop out as soon as possible and endorse Marco Rubio.
The party might not decide
One key reason is that while I, personally, am of the school of thought that outsider candidates like Trump and Carson face much longer odds than their current standing in the polls implies there are no sure things in politics. It's at least conceivable that one of those guys — or Ted Cruz — really could sweep the field. George McGovern and Jimmy Carter, for example, both won Democratic Party nominations over the objections of party insiders.
All the conventional candidates in the field — Bush, Rubio, Chris Christie, John Kasich, and even Bobby Jindal — have something of an obligation to ensure that they don't collectively do so much to divide the establishment that they let an outsider in.
Marco Rubio is Jeb Bush but good at politics
Call it low-energy or blame the lack of support from his mother, but from day one every aspect of Bush's campaign not directly related to fundraising has been unimpressive. Bush has struggled mightily with incredibly banal questions about dynastic politics and his brother's invasion of Iraq, while his arguments in favor of immigration reform have been so weak that the entire nominating contest has been dominated by Trump's racist demagoguery.
Bush is also literally the only person on the planet earth who will be utterly incapable of tapping into a sense that Hillary Clinton's campaign is a bit of a tired retread.
Marco Rubio, by contrast, is a dynamic public speaker and gutsy political risk-taker (recall that he got to the Senate by beating a sitting governor in a primary) who impresses staffers on both sides of the aisle who've worked with him. Rubio performs better than Bush in head-to-head polling against Clinton.
And, crucially, Rubio has the exact same policy positions as Bush — very conservative views on abortion and foreign policy, a shared passion for deficit-increasing tax cuts, and a moderate stance on immigration. Bush was something of a political mentor to Rubio back in Florida, and had Jeb announced a year ago that he simply lacked the fire in the belly for a presidential campaign and endorsed Rubio as a political ally and ideological fellow-traveler nobody would have been shocked.
Sooner is better than later
The people cashing Jeb's checks will, of course, argue that this is all incredibly premature. Bush can keep running for months and always drop out and endorse Rubio later if that's needed as some kind of stop-Trump or stop-Cruz gesture. And just because Rubio is better at politics than Bush (can you imagine someone with Bush's talents making it as far as Rubio has if he'd had Rubio's same modest origins) doesn't mean that Bush can't ultimately bury him under an avalanche of money. After all, several not-very-talented members of Bush's immediate family have gotten to the White House ahead of other, more compelling options.
That said, for months now the Bush campaign has gone nowhere but down. The more people see of Bush, the more they feel "meh" about him. If he quits now for the good of the party, people will say he was a good man driven by a strong sense of duty and noblesse oblige. If he waits for months as his public support continues to bleed away, he'll be humiliated.