Over the past few weeks, the news for Jeb Bush's presidential campaign has been getting grimmer and grimmer:

You will recall, of course, that reports of money problems for Scott Walker's campaign were followed by his withdrawal from the race within days. Bush's Super PAC, however, raised $103 million in the first half of 2015 — so even if current fundraising has slowed, his overall operation still has a lot left in the bank. Still, this is not at all a good place for him to be.

Bush has to crush Rubio or he's toast

The true danger for Bush isn't money, though — it's the prospect that voters and party elites seeking an alternative to Trump will fall behind Marco Rubio. Rubio is, and always has been, Bush's main threat. As I wrote in June:

Sen. Marco Rubio's entrance into the race in April was a serious blow to Bush's hopes — not just because he's also from Florida, but because the approaches the two men take to politics really are quite similar. Unlike bomb-throwers on the right, Rubio consistently frames his quite conservative views in inspiring and appealing terms. Indeed, he's a much better speaker than Bush. And his fresh face and name seem to present a better contrast to Hillary Clinton.

Additionally, Rubio can conceivably trump Bush's claimed ability to appeal to Hispanic voters, and he's much better in touch with today's conservative base compared with Bush, who's been out of politics since 2007. Meanwhile, Bush's family name inextricably ties him to the establishment, and also raises electability concerns considering how unpopular his brother's presidency ended up.

At the outset, Bush could only point to two clear advantages he seemed to have over Rubio: his long record of conservative achievements as governor of Florida, and his ability to raise enormous amounts of money.

Yet Republican voters so far seem completely uninterested in actual governing experience this cycle, as judged by their willingness to send Trump, Ben Carson, and Carly Fiorina to the top of polls — and though Rubio is inexperienced in national politics, he certainly has more than these three. And now Bush's cutbacks show he's having money problems, while Rubio is reportedly close to winning the endorsement of billionaire Sheldon Adelson, who could pour tons of cash into his operation.

Since the rise of Trump and the other outsider candidates, Bush has always been able to hold on to the hope that he'd end up the mainstream, establishment alternative when the dust starts to settle. Even if the early states have some odd results, it seemed that Bush's money and brand name could position him to ride to the establishment's rescue later on.

But now it's looking increasingly like Rubio could fill that role. So Bush's chance of winning going forward depends on him utterly discrediting Rubio. And his Super PAC has tens of millions of dollars to spend to that end.

Of course, Bush could also drop out for the greater good of the Republican Party, as Matt Yglesias suggested last month. It sure seems like the party would have a much easier time beating Trump and the other outsiders if Jeb were out of the picture and elites could unify around Rubio. And it would surely be an impressive example of elite coordination — and a point in favor of the "party decides" theory — if Bush were to quit the race before the early contests.

So if Bush's numbers don't go up in the next few weeks, he'll have to decide whether he wants his team to spend tens of millions of dollars trashing the reputation of a GOP rising star who also might be the party's most electable candidate. For a guy who once said he'd only run for president if he could campaign "joyfully," this could get ugly.

VIDEO: Jeb Bush on Benghazi, 9/11