ES MOINES - The disco beat still rocks the events on Bill Bradley's campaign trail. ''Ain't no stoppin' us now,'' the song blares as he ambles into the room. ''We're on the move.''
But after more than a year on the run, building momentum as he barnstormed from Salem, N.H., to Sioux City, Iowa, Bradley has lost some of the bounce in his campaign step as he approaches a major crossroad on the path to the White House.
Thrust on the defensive by new disclosures about his heart condition and Vice President Al Gore's attacks on his agenda, Bradley yesterday was pressing ahead by bus from Muscatine to Clinton, Iowa, as his strategists sought a dose of political adrenaline to reignite his campaign before Monday's Iowa caucuses and the Feb. 1 New Hampshire primary.
The most recent polls indicate Bradley has stalled in Iowa, where he trails Gore by 20 points, and lost ground in New Hampshire. One tracking poll yesterday showed Bradley slipping behind Gore in the Granite State after protecting a slim, but significant, edge there for several weeks.
''All in all, this is a tough time for Bill Bradley,'' said Stuart Rothenberg, a campaign analyst and editor of the Washington-based Rothenberg Political Report. ''It looks like the momentum he had a month ago has all but evaporated and he's trying to hold on.''
Amid the apparent slippage, Bradley and his staff are searching for reasons, a self-analysis that has generated some disagreement over how to proceed. Most notably, Bradley remains steadfastly opposed to matching the tough ads that Gore has run against him in Iowa.
In addition, there is some concern behind the scenes that Bradley should have responded more vigorously to Gore's most stinging attacks on him, particulary Gore's claim that Bradley showed little concern as a senator for Iowa farmers.
''In politics, you don't do everything perfectly,'' Bradley's spokesman, Eric Hauser, said recently. ''Sometimes, they don't happen the way you want them to.''
A day after disclosing that he had experienced four additional abnormal heart rhythms since an irregularity in December prompted him to seek hospital care, Bradley continued to field queries from voters and reporters about his condition, atrial fibrillation.
His most recent erratic heart rhythm occurred last Sunday in Waterloo, Iowa, on the eve of his final debate with Gore in Iowa.
The first question Bradley fielded at a morning event in Muscatine came from a woman who said she was concerned about his health.
''This is a very common condition, and it has absolutely no impact on your ability to campaign,'' Bradley told the woman, adding that it would not affect his ability to serve as president.
''You should feel reassured on that front,'' Bradley said. ''This is a nuisance for me, and shouldn't be a concern for you.''
The Bradley campaign last night released a statement from one of his cardiologists, Dr. Robert H. Heissenbuttel: ''The natural history of paroxysmal atrial fibrillation includes sporadic increases in frequency of episodes, which does not indicate any change in heart status.... He has been advised to continue all normal activity without limitation.''
But all the attention on Bradley's heart condition has shifted the focus away from his centerpiece issues, such as universal health care, handgun control, and public financing of elections. Rather than advocating racial tolerance, one of his key initiatives, Bradley was largely occupied with medical questions.
His campaign continued to refuse to release fully his medical records, saying that all the relevant information about his heart has already been made public. Aides said Bradley has no other medical condition that is pertinent to his campaign for the presidency.
''He's in very good shape,'' said Bradley's communications director, Anita Dunn. ''What else is there to know?''
Gore, well ahead in the polls here but insisting he will fight for every last vote, downplayed Bradley's heart condition. ''My understanding is that it's nothing serious,'' Gore told reporters after meeting some children at the pediatric section of Des Moines' Mercy Medical Center.
''He's out there campaigning,'' Gore said. ''It's a routine matter. That's the good news.''
Gore recently has carried the confidence of a candidate who has virtually taken the Iowa caucuses, but he is campaigning relentlessly, sometimes making as many as six stops a day, to secure the critical win.
Bradley, by nearly all accounts, is eager to return to New Hampshire on Tuesday, where his candidacy has been far more warmly embraced than in Iowa.
Bradley has worked New Hampshire hard, investing much more money on television ads that are broadcast into the state than Gore. According to a Globe analysis, between Nov. 9 and Jan. 17, Bradley had aired 1,168 minutes of commercials that cost $2.6 million compared with 837 minutes that cost Gore $1.5 million.
Political observers also say they believe Bradley has a more motivated grass-roots organization in New Hampshire than Gore. And Bradley's volunteers first started canvassing the state in July, dropping literature well before the Gore campaign began knocking on doors after Labor Day.
But despite all that, tracking polls in the state have begun to show a downward trend for Bradley and an uptick for Gore.
Now, with questions about Bradley's heart condition, his resistance to releasing medical records, and the likelihood of a large loss in the Iowa caucuses, it's unclear whether Bradley's constant spadework in New Hampshire will come to naught.
But his supporters say they are undaunted. ''If you've been through any of these before, you know that these things tend to ebb and flow,'' said James Shannon, the former Massachusetts congressman who recently spent a weekend knocking on doors for Bradley in Goffstown.
Shannon said he is reminded of the 1992 New Hampshire primary, when many observers thought Bill Clinton would not survive questions about the draft and allegations of an affair with Gennifer Flowers.
''People said, `This is goodbye,''' Shannon said. ''Every campaign runs into these things. You just have to stand firm, keep fighting, and go with your gut. My gut has told me from the beginning that this guy is connecting with people.''
John Rauh, one of Bradley's chief backers in the state, said the disclosures about Bradley's heart only serve to demonstrate his integrity.
''Bill was asked a question, he answered it,'' said Rauh, who believes there is a ''great spirit'' among the volunteers working for Bradley, most of whom are housed by Bradley supporters.
''I think it's very different from the Gore campaign,'' Rauh said of Bradley's youth brigade. ''They're not in it for what they're going to get if he wins. They believe in Bill.''
The Bradley campaign intends to maintain its positive message next week, regardless of the Iowa results, said Mark Longabaugh, the state director. The closing television spots are likely to continue to present Bradley in a straightforward and unadorned manner, either taking questions from voters or giving his stump speech.
Globe Staff writers Michael Crowley, traveling with Bradley, and Susan Milligan, traveling with Gore, contributed to this report.
