Mitt Romney is the favorite to win New Hampshire, according to a Public Policy Polling poll released Tuesday, but Donald Trump, should he enter the race, is nipping at his heels.

The poll found Romney getting at least 31 percent of the vote on a primary ballot where the candidates were Michele Bachmann, Haley Barbour, Newt Gingrich, Mike Huckabee, Sarah Palin, Ron Paul, Tim Pawlenty, and Mitt Romney. But add Donald Trump to the ballot (and remove Haley Barbour, who gets only two percent of the vote on the original ballot), and Romney’s share drops to 27 percent, while Trump takes 21 percent of the vote. Trump does not only take votes from the former Massachusetts Governor; Huckabee and Palin each lose three percent of their original share of the vote if Trump is in the race, and the number of undecided voters drops by 5 percent. Still, he is the only candidate currently within striking distance of Romney, who holds a double-digit lead over everyone else in other ballots.

Romney’s popularity in New Hampshire is not surprising. As the former governor of neighboring Massachusetts, he is well known, and in 2008 he placed second in the state’s primary behind the eventual nominee John McCain. His popularity in the state seems to come in spite of his record, however.

61 percent of voters said that they “would not be willing” to vote for a candidate “who had supported a bill at the state level mandating that voters have health insurance.” This should spell doom for Romney, whose healthcare program as Governor of Massachusetts became the model for President Obama’s despised healthcare bill.

But the poll found that it didn’t actually have much of an effect. Romney’s 68 percent base favorability rating remains the same among voters who would not be willing to vote for a candidate who supported a health care mandate as it does among voters who would be willing to and those who are not sure.

On a sample ballot, Romney still beats out every other candidate, even among those who say they would not be willing to support such a candidate. On the basic ballot in which all candidates are included, Romney polls 31 percent overall, 46 percent from those willing to vote for a candidate who supported a healthcare mandate, and 27 percent from those who would not be willing. So, he gains a lot of points among those who like the idea of a healthcare mandate, but he loses very few points – just 4 percent – among those unwilling to vote for a candidate who supported a healthcare mandate.

There are a couple possible explanations for this. One is that while 61 percent of New Hampshire voters may not be willing to vote for such a candidate on principle, in reality, whether or not the candidate supported a healthcare mandate is not as compelling a factor for those voters as other issues. And when it comes to those other issues, these voters favor Romney.

Alternatively, it could be an issue of messaging. Though the Obama administration has already gone out of their way to associated Romney with the hated health care bill, it may be that this association has not taken significant root among New Hampshire voters. Those who support a health care mandate and care about it know that Romney did something similar as governor and they like that, explaining why he gets so many votes from this group. Among those who wouldn’t be willing to support a candidate that did that, there seems to be little recognition that Romney is such a candidate.

The ‘birther’ movement seems to have taken hold in New Hampshire. A plurality – 42 percent – say that they do not think that President Obama was born in the United States, and 23 percent say they are unsure. 35 percent say that they think that Obama was born in the US. Those who are unsure are at the very least susceptible to the birther argument, which would make that seem like a very compelling line for a candidate that hopes to compete in the state.

Donald Trump has done exactly that, opening questioning the existence of Obama’s birth certificate and the information contained in that document. And according to PPP’s sample ballot, he is in fact the candidate who is the biggest threat to Mitt Romney in the Granite State.

In the ballot without Trump on it, Romney wins. In fact, he polls quite well with those who believe Obama was not born in the country, getting 29 percent of the birther vote and 28 percent of the vote from those who are unsure. Even among this group of people, he beats his nearest opponent by 13 points.

Enter The Donald. When Trump is on the ballot, Romney polls only 27 percent overall, and Trump is close on his heels with 21 percent of the vote. It seems to be a clear-cut case – Trump does well with birthers, two-thirds of the population is susceptible to that line of thinking, and therefore, Trump closes in on Romney if he enters the race. But just because those two facts are true, doesn’t mean that there is necessarily a causal relationship between them.

Trump polls as well as, if not better than, Romney in several demographics. Among those who identify themselves as members of the Tea Party, Trump takes 23 percent of the vote to Romney’s 21 percent – the only ballot configuration in which Romney gets fewer than 28 percent of the Tea Party vote.

Among independents, Trump gets 22 percent to Romney’s 25 percent, edging out Ron Paul, who consisting gets at least 20 percent of the vote from independents in New Hampshire. Romney takes at least 28 percent of the independent vote on every possible ballot that PPP presented to voters. But if Trump enters the race, than Romney’s share drops to only 25 percent.

Trump, in fact, is the only candidate to whom Romney loses independent votes in the possible ballot configurations. Romney starts with a baseline of 28 percent of the independent vote. Remove Huckabee, Palin, or both, and Romney picks up votes. If Rudy Giuliani is added to the ballot, he picks up 16 percent of the independent vote, but he draws votes mostly from Gingrich, Palin, and Ron Paul; Romney still gets 29 percent.

There could certainly be some overlap in all of these groups, but it is safe to say that not all New Hampshire independents polled are birthers, nor are all New Hampshire Tea Partiers. So Trump is not just popular in New Hampshire because he has questioned Obama’s citizenship. That is likely a factor contributing to his popularity, but it is unlikely that it is the only cause.

Furthermore, Trump’s favorability among those who believe that the President was not born in the United States is rather low, relative to other candidates. 51 percent of that demographic said they had a favorable opinion of The Donald. It’s a majority, certainly, but compared to Romney’s 72 percent favorability, Giuliani’s 72 percent favorability or Palin’s 60 percent favorability, it is rather unimpressive. Birthers like Trump about as much as they like Huckabee, who polls 52 percent favorability – but Huckabee doesn’t see any significant lift on the ballot from this demographic.

Moreover, Trump’s unfavorable numbers among birthers are higher than any of the other candidates’. 37 percent say they view him unfavorably. Huckabee is the only other candidate whose unfavorable numbers are even above 30 percent.

Favorability isn’t necessarily an indicator of who people will vote for. Palin and Paul have the third and fourth highest favorability ratings, respectively, after Romney and Giuliani, but they don’t get 3rd or 4th place in the sample ballot. However, Trump’s low favorability numbers from this group of people would suggest that he is not necessarily drawing a tidal wave of support from this demographic.

In this poll, it should be noted that New Hampshire voters appear to be unfamiliar with a number of the possible candidates. More than 50 percent of those polled answered ‘unsure’ when asked if they had a favorable or unfavorable opinion of Haley Barbour, Tim Pawlenty, Mitch Daniels, John Huntsman, Rick Santorum, and Herman Cain. The landscape could shift dramatically as time wears on if any of those candidates, or someone else entirely, gains traction in the state.