MIAMI -- Lawton Chiles, the runaway favorite in the Florida governor's race five months ago, has been thrown on the defensive and is now battling to hold on to a dwindling lead in next month's Democratic primary.

The former U.S. senator, who had hoped to make the campaign a national referendum on clean politics, has stumbled in recent weeks, allowing an attack on his ethics to go unanswered long enough for his primary opponent, Rep. Bill Nelson, to plant seeds of doubt about Chiles's personal finances.

Less than four weeks before the Sept. 4 primary, Chiles, 60, remains on the defensive after revealing that he has resumed taking Prozac, an antidepressant he used last winter to treat a depression that caused moods so bleak he called them "the blacks."

Chiles took the widely prescribed drug last winter but in April, shortly after entering the race, he announced that his depression was cured and that he had stopped taking the drug. Chiles's aides were relieved when, after an initial round of stories about the effects of depression, the headlines about Chiles and Prozac receded.

But with Chiles's admission last week, Prozac was blown back onto the front pages with the force of a hurricane. Both Nelson and Republican Gov. Bob Martinez demanded that Chiles release his medical records, which Chiles refuses to do. And Nelson's running mate, Florida House Speaker Tom Gustafson, offered a politically devastating observation to the editorial board of the Fort Lauderdale Sun-Sentinel: "I don't want to have a suicide during {Chiles's} term of office or during the election."

When he leaped into the governor's race last spring, immediately displacing Nelson as the front-runner, Chiles said he had been inspired by the democracy movement then sweeping Eastern Europe. He challenged his opponents and the media to stick to the issues facing the nation's fourth largest state -- burgeoning growth, overcrowded prisons and dwindling funds to finance education and new roads.

But Chiles also made the campaign and campaign ethics his central issue. He spurned big-money politics, limiting his contributions to $100. He shunned negative advertising. He agreed to debate Nelson, but in the traditional Lincoln-Douglas style. "Mano a mano," Chiles said. He asked voters to judge his performance, his ethics and his health by watching the way he ran his campaign.

His was to be a back-to-the-people movement. The campaign raised $1.7 million from 25,000 contributors. It set up shop in a converted Tallahassee car dealership, and for a while it was a point of pride that it didn't have a campaign manager.

But the approach hurt more than it helped. And Nelson, assured of the Democratic nomination before Chiles entered the race, fought back hard. He invested $2 million in television advertising to both sell himself to Florida voters and to attack Chiles's ethics. By June, Chiles's lead had slipped from 34 points to 17 points, and by late July a poll showed that Chiles's lead had dwindled to 12 points.

Last week, Nelson, 47, was happily campaigning, telling Kiwanis Club luncheon crowds and county courthouse audiences that he was surging. Meanwhile, Chiles found himself fielding questions almost everywhere he went about his campaign flubs, his personal finances and his health.

As Chiles traveled the state last week, he was missing the symbols of his "people's campaign" -- the brightly colored plaid shirt and walking boots. Gone also was the euphoric mood of his springtime lead. Instead, Chiles wore a dark, pin-stripe suit and had a pained and vulnerable look in his eye.

In Miami, after touring a prison, he stepped out into the blazing sun to face a bank of television cameras there to record, not his views of prison overcrowding, but his explanation about his resumption of Prozac.

In Jacksonville, a few days later, after looking over a new county jail, Chiles again faced the local cameras. Topic One was his lone TV spot, rushed onto the air the day before with a misspelled word (innuendo, with one "n") in the text that appears on the screen. The ad contained Chiles's long-awaited defense of his personal finances.

Nelson's attack on Chiles's personal finances produced more smoke than fire, but it put Chiles on the defensive. Over four weeks, Nelson hammered away at several complicated business transactions, calling them "sweetheart deals" and charging that Chiles had failed to fully disclose them when he was in the Senate.

Chiles was caught off guard by the Nelson attack and at times was unable to respond in detail to Nelson's charges. At first, Chiles conceded he may have inadvertently failed to report everything while in the Senate. But he added that he made little or no money on the deals in question. Days later, after his campaign staff researched Senate disclosure rules, Chiles added that he wasn't required to report the transactions in question.

Chiles also pointed out that a Nelson television ad, which suggested that Chiles had made no payments on a loan he received for 11 years, was in error. Chiles produced the checks showing payments were made, but the ad stayed on the air.

Chiles argues that with the huge lead he enjoyed early in the campaign, he had nowhere to go but down. But one campaign insider said recent polls showing his continued slide were "like hitting the wall. It focused our attention." The campaign was reorganized and new aides were added to streamline its daily operations.

But Chiles must still deal with the issue of his health. Chiles's explanation for his resumption of the medication seemed only to make the matter more difficult to defuse. He often equated his depression, and use of Prozac, to diabetes or a thyroid condition that required medication. But Chiles confessed he had gone off Prozac against the advice of his doctor, something he would not have done had he had a thyroid disorder.

Recently, Buddy McKay, candidate for lieutenant governor and Chiles's running mate, watched Chiles slog through yet another grilling about Prozac and shrugged as he offered an explanation for Chiles's handling of the issue. "He's got almost a compulsion to be honest about anything that could be seen as a shortcoming," McKay said.