The first showdown between U.S. Republican nominees in Iowa Tuesday night will be a "very close" race, with former Massachusetts governor Mitt Romney having the upper hand, a former lieutenant governor of the Midwest state told Xinhua Sunday.

In an interview, Patty Judge, who was the lieutenant governor of Iowa until last year, said there is a "very spirited contest" between Republicans in this election, with the frontrunner of the race changing "every few days."

With the exception of Romney, whose ratings have remained steady, GOP candidates took turns taking first or second place in the race. Texas Governor Rick Perry, Minnesota Congresswoman Michele Bachmann, businessman Herman Cain, and former House Speaker Newt Gingrich have all seen their ratings rise and fall.

"It is a very good example of the effect of negative advertising," Judge said. "We have seen leaders have a lot of support behind them, and then we've seen a barrage of very negative TV ads unleashed on that leader, and you can watch just within a few days the number start to fall for that person."

Gingrich was the most recent example of this phenomenon. A month ago the former speaker was unstoppable, polling high near the thirties. But according to the last poll of Iowan caucus goers released Saturday night by the Des Moines Register, Gingrich polled fourth with 12 percent of support.

Romney came out on top in that poll. But he was being challenged by Texas Congressman Ron Paul, who polled 22 percent, only two points behind Romney.

But Judge believed former Pennsylvania senator Rick Santorum, who polled 15 percent in the Des Moines Register poll, is actually an even bigger threat to Romney.

"Santorum is really making a push for second if not first place in this caucus ... in part, that may be due to the fact that there has been almost no negative advertising on Rick Santorum to this point of time," she said.

Judge said Santorum has "stayed under the radar," and "done the work he needed to do by going into the communities and talking to the leadership in the various communities and asking for their support."

With two days left until the caucuses, Judge said, "there's no time to take him down with negative ads."

Santorum has seen a meteoric rise in the past few days from near the bottom of the poll results to near the top. The Des Moines Register poll showed that during the four days of polling between Dec. 27 to 30, 2011, Santorum averaged 10 points after the first two nights of polling, but doubled that during the second two nights. Looking just at the final day of polling, he was just one point down from Romney's 23 percent Friday.

But Judge believed Romney still stands a good chance of winning in Iowa.

"I think it's going to be very close. Mitt Romney is going to get maybe 25 percent (of support)," said Judge, noting he is not likely to get much more than that. Meanwhile, conservatives in the field will be "battling it out" for second place, and Santorum is likely to come in second, she said.

If Romney wins Iowa, he is likely to succeed in New Hampshire, which holds the first primary in the nation about a week later. He is more popular in that state than any other candidate and enjoys strong support there.

In fact, many experts believe that the choice of Republican candidates is unusually limited this year, with Romney being the only one likely to win a bid for the presidency. Judge also suggested that Romney stands the strongest chance of winning the race for the White House, given he wins in Iowa.

"If Mitt Romney comes out of Iowa winning, I think he will go on to win New Hampshire. This Republican nomination process will be wrapped up very quickly with Mitt Romney as their candidate," Judge said.