President Obama and Mitt Romney enter the ring for Round 3

SALT LAKE CITY — Turning the final presidential debate Monday into another dust-up may not be the best way for either President Barack Obama or his GOP challenger, Mitt Romney, to win over undecided voters.

"Two negatives don't make a positive," University of South Florida political science professor Susan MacManus said, warning that a repeat of last week's tense clash could cost both candidates votes.

"This will be their last shot to get their message out to a lot of people," particularly early voters, she said. "If the tone is as negative and un-presidential-like as it was before, many will tune out."

That's especially true in Florida and other swing states, where voters have been subjected to nearly nonstop negative advertising about the candidates since the spring, MacManus said.

If the third presidential debate, being held in Boca Raton, Fla., turns too confrontational, she said voters who still aren't sure whether they'll vote for Obama or Romney may end up not voting at all.

"It could keep the voters that each of them needs away from the polls," MacManus said. Undecided young and women voters "are very alienated by what seems to be a street yard brawl between two guys. They don't like it."

University of Iowa political science professor and Republican activist Tim Hagle said he expects the tone of the final debate to shift because of the subject matter.

"At this point, they've established their debate personas, and with foreign policy stuff, most people think, ‘This is serious,'" Hagle said. "You have to be a little more somber. If you get too aggressive, you look reckless."

Foreign policy, he said, isn't typically a topic that gets voters excited. But this debate may be an exception because of the closeness of the race. In Iowa, also a swing state, Hagle said the race is a toss-up, but the Obama campaign is pushing hard for votes.

Both Republicans and Democrats walked away from last week's debate believing their candidate was the winner. In the first debate, Romney was a clear victor, in part because Obama delivered a lackluster performance.

With early voting going on in many states and Election Day just over two weeks away, Romney is continuing to close in on the president's lead and is now winning in some national and swing state polls. The third debate is seen as one of the final opportunities to impact those numbers.

"This is a crucial event for both campaigns," Colorado State University political science professor Kyle Saunders said. "Both presidential candidates need to win this debate to put the media narrative on their side."

There are some foreign policy issues that voters are likely going to be interested in, including the issues surrounding the president's handling of the U.S. Embassy bombing in Libya, a topic that prompted a sharp exchange between the two candidates in the second debate.

But it's the still-struggling economy that remains the top concern for voters, something that's going to be tricky to bring up in foreign policy debate, Saunders said.

"This topic does not suit that very well at all," he said. "So it's going to be more about how Romney portrays himself on issues that allows people who are still persuadable to decide he could be president."

University of Nevada, Las Vegas political science professor David Damore said the candidates will be judged on how they handle the "disconnect between the topic and what voters are really thinking about."

Damore agreed with Saunders that voters will be more focused on style than substance.

"Clearly, they want substance, but they judge on style," he said.

Foreign policy is not a topic that's reverberating with voters, especially in Nevada, a swing state still reeling from the economic downturn, Damore said.