Republican presidential candidate Jeb Bush speaks at a town hall meeting at the Music Man Square on Sept. 21 in Mason City, Iowa. (Charlie Neibergall/Associated Press)

In April, one in every four likely Republican primary voters nationally said they would support Jeb Bush for president, according to an NBC-Wall Street Journal poll. Today, just 7 percent say the same. And, it's not just the NBC-WSJ poll; here's Bush's trend line since the start of the year:

That's a remarkable — and remarkably steady — erosion of support unmatched by anyone still running for the Republican nomination. The only person to experience anything close to that sort of drastic drop-off was Wisconsin Gov. Scott Walker, who ended his candidacy last week.

So, why isn't everyone talking about Bush's struggles — particularly given that he is the best-known candidate in the field and was widely regarded as the front-runner when he entered the race in June? I put that exact question to some of the smartest minds in the Republican Party who aren't currently working for Bush or any of the other 14 announced candidates.

"Jeb’s strategy is predicated on his remaining the front-runner in the 'political insider' lane," said one longtime GOP operative who spoke on the condition of anonymity to discuss the race candidly. "That is, when the field narrows and establishment Republicans look for an alternative to Donald Trump, Ben Carson or Carly Fiorina, Jeb is counting on being the most popular remaining item on the menu. But that assumption is not something the Bush campaign can count on. [Sen. Marco] Rubio has now eclipsed Jeb as the most popular remaining item on the menu."

Whether or not you agree with that analysis of Bush's prospects, the strategists I talked to were unanimous about one thing: The former Florida governor has avoided full-scale panic over his poll numbers by lapping the other candidates in fundraising — and the campaign organizations that this money can buy. Bush raised $114 million — $103 million of which was for his Right to Rise super PAC — in the first six months of the year, a total that put him head and shoulders above everyone else in the field.

That sort of money edge ensures that Bush (or at least Bush's super PAC) will be able to greatly outspend everyone else this side of Donald Trump on TV ads and organization in early states. (Trump is a billionaire and says he will self fund his campaign but has not indicated how much he has spent or plans to spend.) Bush laid down almost $8 million in reservations for TV time in Iowa, New Hampshire and South Carolina on Monday — the bulk of that planned spending will be in New Hampshire. And Right to Rise has begun a $10 million ad campaign in support of Bush in those same three states that will run through the end of the year.

Bush's ability to sustain his candidacy through not just those three early-voting states but through a process that could be drawn out because of the sheer number of candidates in the race is the thing that keeps the wolves from the door at the moment, said one senior GOP strategist not affiliated with any of the candidates.

"Carson, Fiorina and Trump are not going to be our nominee," the strategist said. "It is a two-man race, Jeb versus Marco. And while I think Marco has a real shot , the idea that they are equal is also ridiculous. Jeb is built for the long haul, which gives him a leg up."

The question that has to keep Bush and his team up at night, however, is: What happens if all of those ads paid for by all of that money don't move Republican voters who have quite clearly not been won over by what Bush has shown on the campaign trail?

"A big chunk of the GOP donor class has so much invested in Jeb Bush’s candidacy," said one high-level unaligned GOP operative. "It turns out a $100 plus million can buy you at least one state and that’s denial." 

That's not a singular sentiment. As my ahead-of-the-curve colleagues Ed O'Keefe and Matea Gold wrote earlier this week in a piece on Jeb's make or break moment:

Right now, the momentum appears to be behind Rubio, who has jumped ahead of Bush in most polls. At least a third of the bundlers who signed up to raise money for [Scott] Walker have switched their allegiance to Rubio, while a smaller number have gone with Bush, according to people familiar with the discussions.

Bush also is facing fresh scrutiny for comments that critics say bear echoes of remarks Mitt Romney made during his 2012 GOP presidential bid, part of a pattern of awkward statements that Bush or his campaign have had to clarify.

The good news for Bush -- such as it is -- is that 55 percent of Republican primary voters in the NBC-WSJ poll said they could see themselves supporting him, while 43 percent said they couldn't. That's not great, but it's actually the fourth-best showing in the GOP field — even as opinions about Bush were, notably, less favorable than they were about Rubio on that question.

The bad news for Bush is that on immigration, an issue that fires up the GOP base like almost no other, he is deeply out of step with the activists. Bush has said repeatedly that he supports some form of comprehensive immigration reform in a party whose base believes it is "amnesty" to let any of the 11 million (or so) people who entered the country illegally stay here. That's to say nothing of his support for Common Core educational standards — a program considered a federal overreach by conservatives — or the dynastic last name that he carries in a nation in a deeply anti-politician mood.

There's an argument to be made that Bush doesn't need to convince the hardcore base of the party that he is one of them — that he simply needs to show that he is the most electable candidate in a final one-on-one contest with the candidate who emerges as the choice of conservatives. But, at the moment, that most electable candidate looks like Rubio — and that is a major problem for Bush.

Now, 125 days still remain between today and Feb. 1, the tentative date of the Iowa caucuses. And, Bush — and his aligned super PAC — have more than $100 million to drop in early states to boost him up and tear down Rubio and his other rivals. But, if they start spending that money and Bush's numbers don't move — or don't move enough — expect panic to set in. And quickly.