It's hard to exaggerate how much is on the line for Hillary Clinton when Iowans head to their caucuses on Monday night.

If Clinton wins, her path to the nomination grows smoother. Even allowing that Sen. Bernie Sanders would probably win New Hampshire's primary in eight days, Clinton would remain well-positioned to beat Sanders in Nevada, South Carolina and into the decisive month of March when the bulk of Democratic delegates are allocated.

Lose on Monday, however, and the best-case scenario for Clinton becomes a protracted state-by-state delegate battle with Sanders that could extend well beyond March. And that's the best-case scenario.

Consider this: If Clinton comes up short against Sanders tonight, she will almost certainly lose New Hampshire to Sanders on Feb. 9. Sanders's average lead over Clinton in polling conducted in the Granite State over the last two weeks is 18 points, according to Real Clear Politics. It's hard to imagine that lead shrinking in the event that Sanders wins Iowa tonight.

The Fix's Chris Cillizza previews the Iowa caucuses, looking at what the outcome could mean for both Democrats and Republicans. (Peter Stevenson/The Washington Post)

Take a step back and think about what that would mean. The former first lady, former secretary of state and former senator from New York — with all of the establishment backing in the party and a massive lead over all comers when she entered the race — would have lost to a septuagenerian democratic socialist from Vermont.

Which brings me to this point: This is not 2008 — for lots of reasons. For one, Clinton never had a lead in national polling (or in Iowa) anywhere close to the one she enjoyed in the early stages of the 2016 campaign. And, she faced a far more crowded field.

More importantly, Sanders is not Obama. Obama, from the time he delivered the keynote address at the 2004 Democratic National Convention, was pegged for great things. While people didn't necessarily expect him to beat Clinton for the nomination, there was a sense that he was a once-in-a-lifetime political talent who might rewrite all of the old rules. Sanders is not those things. He is someone who has spent 25 years in Congress — largely on the outskirts due to his socialist ideology and his deeply liberal ideas. He is someone who announced his presidential bid on the lawn outside of the Senate — with a few dozen reporters in attendance. He is someone, in short, that no one — including Bernie Sanders — thought would make even a semi-serious run at Clinton's dominance.

Sanders's rise, then, is rightly understood as only partially about him at all. What Sanders has done is merely provide a vessel into which a not-insignificant number of Democratic voters can pour their doubts about Clinton's commitment to true liberal principles — or really any set of deeply held values. Sanders's rise is entirely predicated on being the anti-Hillary; he is the non-poll-tested, non-calculating, non-hair-combing vision that many people have for what a politician should be — a "Mr. Smith goes to Washington" if Mr. Smith looked sort of like Larry David and talked like Woody Allen.

To lose to that sort of candidate would be a major psychological blow to Clinton — and the entire establishment that has lined up behind her. (Sanders has not been endorsed by a single U.S. senator.) And it would quickly set off a panic within that same establishment about the prospect of what a Sanders's nomination could mean for down-ballot Democrats, given his unwavering advocacy for tax increases to pay for his health care plan.

Such a panic could — could not would — make it difficult for Clinton to last until primaries in states that are more friendly for her, such as South Carolina on Feb. 27. Talk of trying to add another establishment candidate to the mix — Joe Biden, anyone? — would immediately begin. Clinton could, of course, weather those worries and score a series of victories in South Carolina and beyond. But, as I mentioned above, that sort of drawn-out fight would be Clinton's best-case scenario if she loses Iowa tonight.

The stakes are, therefore, unimaginably high for her. Her future political career could well hang in the balance.