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 yesterday.

Mr. Romney was backed by 30%, ahead of Senator McCain of Arizona with 18% and Mayor Giuliani with 17%, according to the poll in the Des Moines Sunday Register.

Among the Democrats, Mr. Edwards was ahead with 29% of those surveyed, compared to 23% for Senator Obama of Illinois and 21% for Senator Clinton, according to the poll, which was conducted May 12–16.

Mrs. Clinton dismissed the Iowa results during an Associated Press interview yesterday 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 too early to put much stock [in them], especially in a caucus state because you’re never really sure who’s going to show up in January to attend a caucus.”

Mr. Romney’s strong showing is a reward for his focus on Iowa, the Iowa Republican chairman, Ray Hoffmann, said. “He’s been very active here, and he’s got a good organization here,” Mr. 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.”

A veteran Democratic strategist, Ron Parker, had a similar impression of Mr. Edwards’s early poll success. He noted that the strong backing for Mr. Edwards might be surprising to some who assumed the heavy attention being given to Mrs. Clinton and Mr. 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,” Mr. Parker said. “I think you have to get back to the grass roots, 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.

A Des Moines lawyer, Steve Roberts, who sits on the Republican National Committee, said he wasn’t surprised by Mr. Romney’s lead. “He’s obviously put together a strong organization in the state, and he’s had a strong presence in the state,” Mr. Roberts said. “He’s been here more than any of the other major candidates and I think that’s had an effect.”

A Democratic strategist, Matt Paul, said Mr. Romney’s lead in the poll could be due to his decision to launch early television commercials in Iowa, boosting his name identification.

“It’s very good news for political consultants,” Mr. Paul said. “It could have an impact on when others decide to go up.”

Although he supports Mr. Edwards, a Des Moines lawyer, Rob Tully, said the poll was just an indication that the three top Democratic candidates would have to battle for a caucus victory. “The real campaigning hasn’t event begun yet, the house parties and the smaller events where people get a real feel for the candidates,” Mr. Tully said. “It’s way early yet.”

Mr. Hoffmann, the Iowa Republican chairman, cautioned against reading too much into the poll. Such early polling is subject to huge changes in the months leading to the state’s precinct caucuses, tentatively scheduled for January 14, 2008. A high-profile Republican straw poll planned for August 11 could alter those polls and drive some candidates from the race.

“In the last few months, you’ve seen the polls change,” Mr. Hoffmann said. “After the straw poll, things are going to change again.”

Mr. Romney made a splash with a $21 million fund-raising haul in the first three months of the year but has struggled to improve his standing beyond single digits in national popularity polls since then.

Over five months, the former Massachusetts governor has laid down roughly $4 million for TV ads, mainly focusing on Iowa and New Hampshire as he sought to introduce himself to potential voters.