The sparks flew in Thursday night’s GOP presidential debate, no doubt because the pressure is increasing the closer they get to the first primary vote, the Iowa caucuses on February 1. Iowa will likely separate the prime contenders from those on the way out, and the candidates know it.

One of the biggest highlights of the evening was Trump attacking Cruz’s eligibility to be president. Cruz was born in Canada to a mother who was a U.S. citizen and a father who was a Cuban citizen at the time. Cruz responded by pointing out that Trump had no problem with Cruz’s eligibility a few months ago when he wasn’t doing as well in polls — and Trump admitted it. He also noted that Trump’s mother is a Scottish citizen. While there is a debate over what the Constitution means when it says a president must be a “natural born citizen,” that debate has been made largely irrelevant since Obama became president with an allegedly Kenyan father. Of course, there are some in the conservative base who find the ineligibility argument convincing, which could siphon votes away from Cruz.

While Trump retains a significant lead in nationwide polls and in New Hampshire, Cruz is tied with him in Iowa polls. Historically, the eventual nominee has typically won either Iowa or New Hampshire or both, so Iowa looms larger for the candidates than its relatively small population would suggest. A new NBC News/Wall Street Journal poll found that in a hypothetical one-on-one race between Trump and Cruz, Cruz beats Trump 51 percent to 43 percent. When matched one on one with Rubio, Trump takes the lead, 52 percent to 45 percent. When all the current GOP candidates are included, Trump leads the next closest candidate, Cruz, by 13 points.

The reason for the wild fluctuations in the race is in part because no one has emerged as a Ronald Reagan type of candidate for people to fall in love with. Cruz seems to fit the bill politically with stellar conservative credentials, but is lacking Reagan’s charisma. Trump throws a monkey wrench into everything, sounding like Reagan half the time but going overboard with his statements the rest of the time. As a result, there are several solid candidates remaining in the race, leading many to fear no one will win a majority before the nominating convention, which could result in a brokered convention.

Some of the GOP candidates have wildly fluctuated in polling throughout the primary, qualifying for the main debate stage in some debates but relegated to the undercard debate for others. Demoted to the undercard debate last night, Rand Paul angrily chose not to participate at all. Paul has been unable to get any traction, most likely due to running on a non-interventionist platform at a time when terrorism is one of the biggest issues facing the country.

Although one of the least conservative candidates on the GOP side, Chris Christie has generally made up for it with impressive debate performances, which moved him up in the polls and back onto the main debate stage. Outsiders Carly Fiorina and Ben Carson both saw surges, but could not sustain them, and Fiorina was shuffled back to the undercard debate last night. Having never held elected office before, they have a higher hurdle to overcome and are apparently having trouble clearing it with many GOP voters.

Considered even more moderate than Bush, John Kasich has not performed well, but somehow has still managed to stay in the top tier debates. Bush began going aggressively after Trump in recent debates, but he lacks the passion necessary to make up for his establishment reputation. Mike Huckabee has been unable to get any traction, despite performing fairly well in the debates, and was relegated to the undercard debate last night. Rick Santorum remains on the undercard, unable thus far to gain traction with his combination of social conservatism and an economic plan seemingly well-suited to court rust belt and blue collar voters.

Many Republicans are panicking that Trump may be the nominee, and are trying to figure out how to stop him. Others have resigned themselves to the fact he may be the guy. However, if enough Republican candidates drop out, it could tip the race to someone like Cruz. Huckabee has said he will likely withdraw if he does poorly in Iowa. If a couple more candidates also withdraw, then the heated exchanges we saw between Trump and Cruz Thursday night may be just the beginning.