DES MOINES, Iowa — Republican Mitt Romney and Democrat John Edwards are atop a poll of likely Iowa caucus voters because both presidential candidates have invested so much time organizing and visiting the state, activists in both parties said Monday.

Romney was backed by 30 percent, ahead of Arizona Sen. John McCain with 18 percent and former New York City Mayor Rudy Giuliani with 17 percent, according to the poll in the Des Moines Sunday Register.

Among the Democrats, Edwards was ahead with 29 percent of those surveyed, compared to 23 percent for Illinois Sen. Barack Obama and 21 percent for New York Sen. Hillary Rodham Clinton, according to the poll, which was conducted May 12-16.

Clinton dismissed the Iowa results during an Associated Press interview Monday in Florida.

"There have been so many polls in Iowa, as there have been in every other state," she said. "In the last week, there have been polls where I've been ahead, polls where I've been second, polls where I've been in third. I just think it's way to early to put much stock (in them), especially in a caucus state, because you're never really sure whose going to show up in January to attend a caucus."

Romney's strong showing is a reward for his focus on Iowa, said Iowa Republican Chairman Ray Hoffmann.

"He's been very active here, and he's got a good organization here," Hoffmann said. "Every election cycle, you can see that people who spend a lot of time here, people who have a good organization, their polls look good."

Veteran Democratic strategist Ron Parker had a similar impression of Edwards' early poll success. He noted that the strong backing for Edwards might be surprising to some who assumed the heavy attention being given to Clinton and Obama would vault them into the lead.

"I think there were folks out there who assumed the buzz was enough to create a front-runner," said Parker. "I think you have to get back to the grassroots and this shows you can't win the Iowa caucuses on name ID and buzz alone."

The Register's poll was conducted by Des Moines-based Selzer & Co., based on telephone interviews with 400 voters who said they likely would attend Democratic caucuses and 401 voters who said they likely would attend Republican caucuses. The poll claimed a margin of error of 4.9 percentage points.

Des Moines lawyer Steve Roberts, who sits on the Republican National Committee, said he wasn't surprised by Romney's lead.

"He's obviously put together a strong organization in the state and he's had a strong presence in the state," Roberts said. "He's been here more than any of the other major candidates and I think that's had an effect."