Jeb Bush (Jonathan Ernst/Reuters)

In an interview with NBC's Peter Alexander on Tuesday, Jeb Bush resisted the idea that Saturday's South Carolina primary is a do-or-die moment for his presidential hopes.

"No it isn't," he said. "The obituaries have been written probably once a week, and we are in it for the long haul. But we are going to do well here."

That's a nice, positive sentiment. It's also totally wrong.

The simple facts are these:

1. Jeb Bush entered this race as the clear Republican front-runner.

2. A super PAC aligned with his candidacy raised more than $100 million in the first six months of 2015.

3. There have been two actual votes in the election thus far. Jeb Bush has finished sixth and fourth in those votes. In Iowa's caucuses, he got 2.8 percent (5,238 votes). In New Hampshire, he received 11 percent (31,310 votes).

If Bush didn't come in ahead of Marco Rubio in New Hampshire — he beat the Florida senator by 1,278 votes — it's hard to see how he even makes it to South Carolina. As it is, he is clinging by his fingernails in this race.

And the Bush people have gone out of their way to insist that South Carolina is "Bush Country." (I wrote a piece on how that might be a miscalculation here.) Bush's brother, former president George W. Bush, and his mother, Barbara, have been on the campaign trail in South Carolina for him in the last few days.

When you do all of that, you set expectations. And those expectations aren't to "do well" — whatever that means. Bush needs a top three finish badly. And even more badly, he needs to finish in front of Rubio.

Short of that, it's very hard to imagine how Bush can — or should — carry on.  The next state to vote is Nevada, which will hold its Republican caucuses Feb. 23.  A new CNN/Opinion Research poll released Wednesday put Bush at 1 percent in the state — 44 points behind Donald Trump and 18 points behind Rubio. Bush is also one point above zero.

After Nevada, there isn't another vote until March 1 — the so-called "SEC primary." The bulk of states voting that day — as you might have figured out by the name — are in the South.  Alabama, Georgia, Tennessee and Texas are all states where people like Trump, Rubio and Texas Sen. Ted Cruz are far better positioned to win, place or show than is Bush.

Then there is the reality that the establishment's only chance — and it's not even that great a chance — to push its way into the top tier with Trump and Cruz is to unite behind a single candidate. New Hampshire's results made that process more difficult. But with Trump looking like he is going to run away with South Carolina, the pressure on the establishment candidates is even higher now.

Bush — and his family — are the beating heart of the Republican establishment. That status worked in his favor in the early days of the race (see point #2 above) but now begins to function as an anchor around his neck. If Bush finishes fourth or even fifth (some polling has him behind Ohio Gov. John Kasich) in South Carolina, there will be an immediate move by elected officials and major donors to find Bush an exit strategy in advance of Nevada in hopes of making Rubio ascendant.

Now Bush, of course, can ignore those calls. He can stay in the race through Nevada, through March 1 and all the way until March 15 when Florida votes. The question is what sort of campaign he could run if he didn't make it into the top three in South Carolina and how staying in the race would impact the overall perception of his political career.

Hanging on in the race just to do it isn't likely the last impression Jeb wants to leave on Republicans and the country more broadly. While it might gall him to endorse Rubio — his one-time mentee — that would be the "right" thing to do for the party. And Jeb, say what you will about him as a candidate, has shown himself through the years as a loyal GOP foot soldier.

South Carolina is the whole shebang for Jeb.  His entire political life comes down to whether he can find a way to get past Rubio and Kasich on Saturday night. If he can't do that, his campaign is effectively over — no matter what he says.