When Jeb Bush announced he was exploring a presidential run last December, he quickly became, in the eyes of many, the closest thing the GOP field had to a frontrunner. He took the lead in polls, deterred Mitt Romney from challenging him, and hoovered up more than $100 million — a sum the other candidates couldn't hope to match.

Now people are wondering if he'll even make it to the Iowa caucuses.

Bush is polling poorly. He's slashing his campaign's payroll and budget. Endorsements of his campaign have slowed to a trickle. And one of his fundraisers told the Washington Post last week that the campaign "feels very much like a death spiral," adding, "I wouldn’t be shocked in 60 days from now if he wasn’t in the race."

During a campaign appearance Saturday, Bush sounded a bitter note. "I've got a lot of really cool things I could do other than sit around, being miserable, listening to people demonize me and me feeling compelled to demonize them," he said. He was referring to Washington gridlock and not his campaign woes — until he added, "Elect Trump if you want that."

Trump is leading the polls and frequently spews insults at Bush, so it makes sense that many blame (or credit) him for Bush's woes. But he's not truly the culprit.

First, Bush's candidacy had serious weaknesses that were evident long before Trump arrived on the scene. And second, the person who has the most potential to knock Bush out for good isn't the fire-breathing reality TV star — it's Bush's home-state rival, Marco Rubio. Because if Rubio catches on, and convinces the establishment to back him, that's likely the end for Jeb.

Bush looked like a really weak frontrunner from the start

Jeb's plan was to start 2015 with a show of strength — a "shock and awe" fundraising drive that would both raise a record-breaking amount of money and convince Republicans to fall behind him like they did for his brother in 1999.

The first part of that happened, but the second didn't. Neither the Republican Party nor its voters rallied to Bush's side.

As far as the party goes, it's quite clear that a great many people in the GOP's fundraising class decided to throw their lot in with Bush, considering how much money he raised. But elected officials were always far more hesitant to do so — according to FiveThirtyEight's tracker, they've been endorsing at their slowest rate in decades. Bush has a narrow lead out of the field, but his numbers are so weak — zero governors, three senators, 10 members of Congress who aren't from Florida —that it hardly matters. He didn't come anywhere close to where his brother was.

As for voters, in all that time in the first half of the year when Bush had a small lead, he never managed to hit even 15 percent support in the HuffPost Pollster national primary average. That's a dreadful performance for a frontrunner — especially one who has high name recognition thanks to being a former president's brother (and son).

So it was clear that despite his boffo fundraising, Bush was having trouble making the sale to both voters and party elites well before Trump jumped in at the end of June. Part of this is because he had been out of politics since his term as governor of Florida ended in January 2007. And while he was off making money, he missed all of the battles of the Obama era that helped shape today's conservatives. So it's understandable that when he parachuted in eight years later to run for president, voters responded by asking, "I'm sorry ... who are you again?"

The GOP's membership had changed too. A huge number of today's Republican elected officials were elected during the anti-Obama or Tea Party waves. Many of them owed little or nothing to the Bush family, or indeed blamed the failure of George W. Bush's presidency for Obama's election. "The notion that Jeb Bush is going to be the Republican presidential nominee is a fantasy nourished by the people who used to run the Republican Party," BuzzFeed's Ben Smith wrote a year and a half ago.

Bush has stagnated in polls and is now disliked by GOP voters

So by the end of June, it was clear that Bush wasn't going to be handed the nomination. The good news for him back then was that nobody else was doing any better.

Enter Trump. The controversial billionaire began dominating headlines and shot well above Bush in the polls. He also began a campaign of insults aimed at Bush — calling him "low-energy," a creature of his donors, and "weak on immigration." Another political neophyte, Ben Carson, soon surged to second place.

Now, the story of polling in the GOP race so far is the failure of any actual politician to catch on. As grim as things are for Bush, he remains effectively even with Rubio in polls, drawing support in the high single digits both nationally and in New Hampshire. Every other current or former elected official except Ted Cruz — a politician despised by every other politician — is even lower. So when they're all doing so badly, I'm skeptical that it's traits unique to Bush, like his gaffes or his immigration views, that are preventing him from catching fire.

Still, it does appear that Bush has been uniquely wounded by the summer and now autumn of Trump. A recent Monmouth national primary poll shows that Bush now has the lowest "favorable" rating and the highest "unfavorable" rating of the top-polling GOP candidates. Check out this chart:

Favorability numbers can change in a hurry — Bush's were far more positive in July, according to Monmouth, and Trump's went from highly negative to highly positive — so part of Bush's decline may just be because he's been the most frequent target of Trump's insults.

But this could also potentially herald a more decisive rejection of Jeb Bush by a plurality of Republican voters. In a year when outsiders are doing well, he is, after all, inextricably tied to the party establishment due to his last name. And another recent poll found that Trump would win a head-to-head primary matchup against him, 59 to 41.

Furthermore, small donors in the party have shown little desire to contribute money to Bush's campaign. A recent National Review article written by a pseudonymous Republican policy adviser called Bush's small donor fundraising "abysmal," pointed out that his "big-donor to small-donor ratio is 15:1," and argued that Bush's supporters are "maxed out" (having already given the maximum allowable contribution to his campaign) and that "he has no grassroots support to grow new ones."

VIDEO: Donald Trump is trolling Jeb Bush

The biggest danger for Bush is that the establishment will back Rubio

Now, there are still another three months before the voting actually begins, and past primary polls around this time have frequently failed to point to the actual winner. Things can change.

But these polls still matter to the extent that they affect the perceptions and strategic choices of party actors now. And more and more of these establishment actors are concluding that Bush's campaign is failing to meet their expectations. Money has reportedly been coming in more slowly, and Bush has only been endorsed by two members of Congress in the past two months.

Indeed, Bush's campaign is being judged so harshly partly because Republican moneyed elites have invested so much into it already — and they want to see some progress. After all, Bush's Super PAC has already spent more than $10 million on TV ads — nearly twice as much as any other candidate, according to NBC News and SMG Delta — to no apparent benefit.

So when Michael Bender and Mark Halperin of Bloomberg Politics report that Bush's team has been "under pressure from their donors" — pressure that led them to slash salaries and cut back the campaign's spending last week — this should be interpreted as Bush's donors expressing some real doubts about the viability of his candidacy. Mitt Romney faced similar doubts from the party's establishment in 2011 as Herman Cain and Newt Gingrich surged past him in the polls. Back then, though, the party didn't really have any other choice but to stick with him (particularly after Rick Perry flamed out).

The difference is that this year there's Marco Rubio, who seems like an obvious choice for an alternative establishment standard-bearer.

In the judgment of political elites, Rubio has done better than Bush in both debates and has made fewer gaffes during the campaign. He's a better speaker and more charismatic. He has closer ties to the modern conservative base. Polls have shown him beating Trump head to head. And he has a fresher face and name that might make him not only a stronger primary candidate than Bush but a more formidable general election candidate too.

Party elites haven't rallied to Rubio so far, and I have my own doubts about whether they can necessarily determine the outcome of the nomination contest. But it does seem clear that Bush's own path to victory relies on consolidating them and the voters who are likely to be sympathetic to them. And right now they're wondering whether going with Rubio might be a better option.

That's why the very small polling bump for Marco Rubio, not the soaring numbers of Trump and Carson, is Bush's biggest threat. Because if Bush loses the establishment support he already has, and the many of its members that remain on the fence, to Rubio — he's through.

VIDEO: Jeb Bush explains the difference between Benghazi and 9/11