The Des Moines Register released its final poll on Saturday. For Republicans, Donald Trump still leads (28%) the horserace, with Ted Cruz close behind (23%). Marco Rubio has moved up to third (15%). The pollster, J. Ann Selzer has some caveats. She argues that while Trump leads, Cruz seems more consistently aligned with the electorate. Her poll, by her own admission, may be underestimating evangelical turnout. If Evangelicals vote in rates similar to 2012, the race will be tight. If Cruz wins, this will be the most likely explanation.

Trends are also of interest. Cruz is still in decline compared to recent polls. Rubio has surged among mainstream Republicans, but a plurality of them still favor Trump, so even in Rubio’s area of strength, he has not been able to unseat his rival. Ben Carson, who once led in Iowa, is still sliding. He is now at 10%. At the same time, Rubio and Carson are the most liked candidates and their supporters are the most enthusiastic (though Trump’s supporters are nearly as enthusiastic). In a caucus setting, enthusiasm matters because the process is more involved, takes more commitment, and involves some level of public proclamation. Whatever your experience is with primaries, they are not caucuses by any measure.

So, what might happen and how will it matter?

1. Trump underperforms compared to expectations. This could happen because Trump is such an unusual political phenomenon. Polls could be overestimating his strength because he has attracted unconventional supporters who may be less likely to vote. If this happens, Trump could slide to second and lose to Cruz, which would not necessarily be devastating. The worst case scenario for Trump would be a third place finish behind both Cruz and Rubio.
2. Trump overperforms compared to expectations. This could happen, again, because Trump is unusual. Many polls screen for likely voters by asking if the respondent voted in recent elections. If the respondent says ‘no,’ the interviewer often concludes the interview at that point. If a fair chunk of Trump’s supporters fit this category, he may perform better than we think and stomp on his rivals. If he does this in Iowa, and follows that up with a victory in New Hampshire, the race would officially be his to lose at that point. Nothing would be guaranteed, but Trump would build even more momentum and his nomination campaign would step toward inevitability.
3. Cruz loses Iowa. The past tells us the candidates most attractive to evangelical Christians do well in Iowa. Mike Huckabee and Rick Santorum are recent examples. Pat Robertson finished second in Iowa in 1988, ahead of eventual nominee George H.W. Bush. If Cruz loses in Iowa, his path forward gets far more rocky. An inability to win in Iowa may signal an inability to win elsewhere.
4. Cruz wins Iowa. If Cruz wins in Iowa, it is not necessarily a signal of momentum going forward. George H.W. Bush won Iowa in 1980. Bob Dole won Iowa in 1988. Like Huckabee and Santorum, neither were able to build enough on Iowa wins to earn the party nomination. Winning Iowa guarantees nothing. At the same time, George W. Bush won Iowa in 2000 and Bob Dole won it again in 1996. Both went on to secure the Republican nomination. A Cruz win would matter, but how much? That remains to be seen.
5. Rubio outperforms expectations. If Rubio finishes a strong, and close, third, or a shocking second or first, his campaign will leave Iowa with some evidence that he may be the last anti-Trump candidate standing. If so, it is possible he will begin to peel away Bush, Christie, Carson, Fiorina, and Kasich supporters and begin to mount a stronger challenge to Trump or Cruz.
6. Rubio underperforms compared to expectations. If Rubio does poorly, a far, distant third, or a fourth while Carson takes third, his narrative could begin to unravel. He would no longer look like the most potent challenger to Trump and Cruz. Perhaps that would benefit Christie or Kasich in New Hampshire and one of them could emerge as the viable alternative.

Obviously, none of these scenarios happen in a vacuum. They are all related to one another, so if Trump ascends, others will descend by definition. More than general elections, presidential primaries are about momentum–the ability to generate growing support, which increases media exposure, poll numbers, funding, and, eventually, votes. Once a nomination picks up enough steam, the bandwagon effect begins and people drift toward the perceived winner. Iowa does not, by itself, provide momentum, but it is the first spark that might lead to a flame. Therefore, it is worth watching.