Donald Trump, Ron DeSantis, and Nikki Haley.
CNN  — 

Former President Donald Trump looks to be on the verge of a historic victory in the Iowa caucuses. The final poll from Iowa’s premier pollster, Ann Selzer, has Trump at 48% followed by former South Carolina Gov. Nikki Haley at 20% and Florida Gov. Ron DeSantis at 16%.

If the final outcome Monday night mirrors the poll, Trump will have won the highest Iowa GOP caucus vote share for a non-incumbent ever.

Still, his victory is not the only thing that would matter. What ultimately may be more important in the short term is his margin of victory – and who finishes in second place.

That’s because with New Hampshire voting in just 10 days, expectations – and whether candidates meet them in Iowa – matter.

Expectations may seem like some immeasurable metric made up by a bunch of pundits. They’re not. Expectations are easily measurable. It’s about how a candidate does in the voting relative to their final polls.

Polling in the Granite State shows the race close with Trump and Haley tied, once you reallocate supporters of former New Jersey Gov. Chris Christie – who dropped out of the race last week – to their second choice, according to CNN’s most recent poll conducted by the University of New Hampshire.

There are two variables that historically best predict New Hampshire primary results at this point: the New Hampshire polls (like the one I just mentioned) and how well a candidate does in Iowa compared to their final polls.

This may seem a little odd, but it actually makes a lot of sense. When someone exceeds expectations, they receive a rush of good media coverage. When someone underperforms, the opposite happens.

Consider what happened to George W. Bush in 2000, who currently holds the record for the best non-incumbent Iowa GOP caucuses performance (41%). He didn’t get any bump out of that win because his margin over his opponent Steve Forbes was considerably smaller than expected.

Bush would go on to lose the New Hampshire primary by a wider-than-expected margin to John McCain, who was already ahead in the Granite State and didn’t contest Iowa.

Four years earlier, Patrick Buchanan nearly shocked Bob Dole in the Iowa GOP caucuses. Buchanan had one of the larger Iowa overperformances of all time. He then was able to mount a comeback and beat Dole in New Hampshire.

Neither McCain nor Buchanan ever really came close in the national polls to the front-runners.

Gary Hart did, however, on the Democratic side in 1984 – even though he lost Iowa by over 30 points to Walter Mondale. Hart did better than the pre-caucus polls and was able to turn that into positive media attention. He then mounted a comeback in New Hampshire, won there and the race was on.

Haley isn’t down by anywhere near the margin Hart was in the New Hampshire polls at this point – well into the double-digits. Even the slightest overperformance by her or the slightest underperformance by Trump in Iowa could shift the New Hampshire race in her direction.

A Haley win in the first primary race in the nation would indicate that Trump is not invincible. Then she’d have a month to try and win the primary in her home state of South Carolina.

On the other hand, it’s easy to imagine the opposite happening. Haley wants to make this a campaign between Trump and her. That won’t happen if DeSantis comes in second place.

For the Florida governor, second place would provide a real reason to stay in the race. If he comes in third in Iowa, then he’s looking at a very tough road ahead.

He’s polling in fourth place in New Hampshire, a distant third place in South Carolina and is 50 points behind Trump nationally. If DeSantis can’t come in second place in Iowa, where can he come in second or even first place?

For Trump, the benchmark is clear. Al Gore remains the only non-incumbent to win all 50 states in either the Democratic or Republican primary for president. Like Trump, his weakest state was New Hampshire.

Gore kept Bill Bradley from gaining too much momentum in New Hampshire with his Iowa performance. He did more than 5 points better than where his final Iowa poll had him. (His 28-point margin over Bradley was the same as the final poll found, as both Bradley and Gore won some undecided voters.)

The Gore comparison may be an apt one for Trump. Gore is the only candidate who has consistently polled equal to or better than Trump in Iowa. Likewise, he is the only candidate besides Trump who led in basically every poll of every early state and nationally.

This latter point is especially important for Trump. Because, for as much as Iowa and New Hampshire matter, the only true objective in the primary season is to win the nomination.

If you’re a student of Iowa results, you know that the winner of Iowa’s Republican caucuses rarely goes on to be the nominee. Only two non-incumbents have done so: Dole in 1996 and Bush in 2000. Every other time, the New Hampshire winner has done so on the Republican side.

The fact that 1996 and 2000 were exceptions to the rule is no accident. Both of those years were the only two times when the Iowa winner was the same as the leader in national primary polls.

Right now, Trump is leading in those national primary polls by 50 points. Because the former president is so strong, he’ll probably be able to withstand a New Hampshire loss.

This puts all the more pressure on his rivals to do something to upend expectations in Iowa. If they want to put any dent in his cloak of invincibility, it starts Monday night.