DES MOINES, Iowa — Bernie Sanders' momentum is shaping the final hours of the race to win tonight's Iowa caucuses, forcing his rivals to lower expectations and feeding the Democratic establishment's fears about what a Sanders victory could do to the party.

The state of play: Advisers to Joe Biden and Pete Buttigieg are already insisting that Iowa's not everything in advance of possible disappointments tonight. "We view Iowa as the beginning, not the end," Biden adviser Symone Sanders said Sunday.

Buttigieg's aides say he doesn't have to win Iowa to be the nominee.

And John Kerry, a Biden supporter and the 2004 Democratic presidential nominee, was overheard by NBC News sounding the alarm on a phone call about ""the possibility of Bernie Sanders taking down the Democratic Party."

What we're seeing: Biden looked tired at his closing rallies. The crowds are older. There's a sense from people who know him that the impeachment focus on his son Hunter has taken a toll.

The questions that reporters ask his aides and surrogates often assume that he won’t win, and that it will be worse than expected.

The big picture: There's a lot of turmoil ahead in the first contest in the race to become President Trump's Democratic opponent — including the apparent Sanders surge and new rules that could inject early results into the caucusing dynamics.

Reality check: Iowa wins have propelled long-shot Democrats like Jimmy Carter in 1976 and Barack Obama in 2008, and showed how enthusiasm can turn into lasting momentum for close seconds, like Sanders in 2016.

But Donald Trump in 2016 and Bill Clinton in 1992 won the presidency without winning Iowa.

What to watch: There will be a lot of noise coming out of tonight's contest, but these are the three things that will matter in the caucuses: Raw vote totals, delegates, and turnout.

For the first time, we'll see raw vote totals from both the first and second alignments.

The raw vote total that really matters will be the one that's reported with the second alignment because not every candidate will reach viability (15% support) in the first alignment and their supporters will have the opportunity to caucus for someone else.

While candidates will likely try to use these numbers to claim a victory of some sort, the real winner of the Iowa caucuses is and always has been based on the number of state delegate equivalents they pick up.

But early speculation has focused on whether Sanders, if his numbers look high at the outset, declares victory before the caucusing is complete, impacting attendance and preferences. How to respond if anyone declares an early victory has been a subject of debate inside rival campaigns.

In 2016, Hillary Clinton won the caucuses with 49.9% of state delegate equivalents while Sanders narrowly lost, earning 49.6% of state delegate equivalents.

Democratic campaigns and operatives have been predicting turnout at this year's caucuses will be somewhere between 2008 (239,000) and 2016 (171,000) — but it's really about the makeup of those voters.

Higher turnout helped Obama edge out Clinton in 2008.

In a campaign event on Saturday, Sanders told supporters that if voter turnout is low, "we're going to lose — simple as that." But, he added: "If they come out in large numbers, we're going to win this caucus."

While an increase in turnout overall is expected from 2016, it's important to look at who turned out: liberal or moderate voters; younger or older voters; and of course the share of non-white voters who caucus and for whom in a mostly-white state.

Only 6% of Iowans are Latino, but Sanders more than any other 2020 Democrat has been working overtime to court them and get them prepared to caucus for him on Monday night.

“We have been preparing for the highest caucus turnout in our party’s history,” Iowa Democratic Party Chair Troy Price said recently.

Go deeper: Democrats take on Trump, each other ahead of Iowa caucuses