One of this year's candidates for president, to hear his opposition tell it, has a long history of policy reversals and rhetorical about-faces -- a zigzag trail that proves his willingness to massage positions and even switch sides when politically convenient.

The flip-flopper, Democrats say, is President Bush. Over the past four years, he abandoned positions on issues such as how to regulate air pollution or whether states should be allowed to sanction same-sex marriage. He changed his mind about the merits of creating the Homeland Security Department, and made a major exception to his stance on free trade by agreeing to tariffs on steel. After resisting, the president yielded to pressure in supporting an independent commission to study policy failures preceding the Sept. 11, 2001, attacks. Bush did the same with questions about whether he would allow his national security adviser to testify, or whether he would answer commissioners' questions for only an hour, or for as long they needed.

Democrats working for John F. Kerry cite these twists and turns with glee -- but even more frustration. Polls have shown overwhelmingly that Kerry -- with his long trail of confusing and sometimes contradictory statements, especially on Iraq -- is this year's flip-flopper in the public mind, a criticism that continued to echo across the campaign trail yesterday.

Once such a popular perception becomes fixed, public opinion experts and strategists say, virtually every episode in the campaign is viewed through that prism, while facts that do not fit with existing assumptions -- such as Bush's history of policy shifts -- do not have much impact in the political debate.

Why these impressions became so firmly fixed in the first place is a source of debate. Bush strategists say the popular perception is true. The president's principles on such issues as low taxes and confronting overseas threats are not in doubt, no matter some occasional tactical shifts, they say, while Kerry's maneuvering on Iraq and other issues raises questions about whether he can stand steady for core beliefs.

Kerry defenders say the flip-flop charge has resonated through purposeful repetition by the Bush campaign, which began striking the theme in ads in the spring and has never let go. In the latest Bush campaign spot, released yesterday, Kerry is shown windsurfing as the ad, scored with Johann Strauss Jr.'s "Blue Danube" waltz, says the Democrat shifts positions on Iraq, health care and education "whichever way the wind blows."

As Democrats see it, the flip-flopper allegation is this year's equivalent of how the GOP four years ago portrayed Al Gore as a chronic truth-stretcher, and now, as then, blame the news media for accepting and promoting a caricature.

For a while this summer, Kerry's team tried to answer Bush's charge that Kerry is equivocating and inconstant by alleging that Bush is just as much or more so. But lately the campaign has laid off this line of argument after concluding it was ineffective against an opponent who surveys show is seen by a majority of voters as decisive, even to the point of stubbornness.

"When it comes to shifting positions, he can shift with the best," Kerry spokesman Joe Lockhart said of Bush. "We are prosecuting a different case. We are not arguing that he's a flip-flopper -- he is -- but that the policy choices he has taken have failed miserably."

Andrew Kohut, director of the Pew Research Center, said voters' perceptions are too settled at this point to allow easily for alternate arguments. Once the public concludes that a politician is strong in general, he or she has more freedom to be flexible on certain delicate particulars, Kohut said, adding that in the case of this year's nominees, "Bush can get away with a little more, and Kerry can get away with a little less."

Stuart Stevens, one of Bush's media advisers, argued that the public's judgments are fair. Voters do not believe that Bush has never changed his mind, or penalize Kerry for every change of position, he said, but over time have reached conclusions about both men's priorities and leadership styles.

"I think all these issues resonate with any candidate if they strike people as part of a pattern and reasonable and true," Stevens said. "The president is someone who has a core set of beliefs and values that are guiding him as he tries to make decisions.

"I don't think he hesitates to change an approach if he feels it's not working, but I don't think people sense remotely that he's doing it [based on] a political compass," he added.

The record, however, suggests a fair degree of political calculation has gone into some of Bush's about-faces. During his first term, the paramount goals -- such as cutting taxes or pursuing a confrontation with Saddam Hussein -- have been fixed. But this has allowed room for tactical maneuvering on other questions.

In 2000, Bush said he would include carbon dioxide on a list of air pollutants requiring federal oversight, a stand he abandoned within weeks of taking office. A month after the Sept. 11 attacks, Bush's spokesman said the president believed a homeland security department that Democrats proposed was "just not necessary." A year after that, Bush had switched course and was lashing some Democrats for not moving quickly enough to approve the agency.

While Bush professes himself a strong free-trader, most other free-trade proponents said he bent on principle in March 2002 when he ordered tariffs on imported steel -- a move that resonated politically in electorally important industrial states such as Pennsylvania. Facing an escalating global trade dispute, he lifted the tariffs at the end of last year.

In some cases, Democrats say, Bush's position stays the same even as his reasons flip. The most famous examples involve taxes and Iraq. He supported tax cuts in 2000 because he said they were affordable in a time of large government surpluses, and once in power he supported them amid rising deficits because he said the economy needed stimulation. The president's principal rationale for the Iraq invasion was to end Baghdad's suspected mass-weapons program and links to international terrorism. In the absence of compelling evidence of these, the main post-invasion rationale has been to rescue Iraq from a tyrant and support democracy in the greater Middle East.

Iraq, however, has been the source of the most damaging charges of equivocation and wind-shifting against Kerry. The Massachusetts senator voted for the Iraq war in October 2002, but a year later voted against Bush's request for $87 billion for military and reconstruction spending in Iraq and Afghanistan. The latter vote came when former Vermont governor Howard Dean's antiwar candidacy was ascendant. The vote may have been wise politics at the time, but came with a high price -- lending an aura of plausibility to the subsequent charges by Bush that Kerry is motivated by opportunism.

Kerry's statements have compounded the damage. In September 2003, he said at a Democratic debate, "We should not send more American troops" to Iraq. "That would be the worst thing." In April, he said on NBC's "Meet the Press" that "if it requires more troops . . . that's what you have to do." In August, he told ABC's "This Week" that if elected, "I will have significant, enormous reduction in the level of troops." This week, he said that, as president, he would not have launched an invasion if he had known that there was not clear evidence of weapons of mass destruction or ties to al Qaeda, though last month he said, knowing these things, he still would have voted to give Bush congressional authority to wage the Iraq war.

Polls make clear the extent to which Bush's flip-flop charge has stuck. A poll released last week by Kohut's Pew Center showed that 53 percent of voters believe Kerry "changes his mind too much." This was down a few percentage points from a poll the week earlier, apparently showing that the effects of the Republican National Convention -- in which delegates swayed in unison chanting "flip-flop, flip-flop" about Kerry -- are wearing off. Even so, the latest data show that 62 percent said the attribute "takes a stand" applies more to Bush than to Kerry, while 29 percent said the opposite. Bush won by 57 percent to 34 percent on which candidate more deserves the phrase "strong leader."

Rep. Rahm Emanuel (D-Ill.) blames the news media for such perceptions. "Journalists decide a type, and then write to type," he said. "Gore as the 'exaggerator.' John Kerry has not done anything that George Bush has not done in spades, but you guys all decide he's 'resolute.' "

Stevens, who has been studying Kerry since advising then-Massachusetts Gov. William H. Weld (R) in an unsuccessful attempt to defeat the senator in 1996, said Kerry's very manner exacerbates the flip-flop impression: "He says these things with great condescension, [suggesting]: 'If only you were as smart as I and understand this that these issues are too complicated to have a consistent position.' . . . People have a good internal detector of the difference between nuance and confusion and opportunism."

John F. Kerry is shown windsurfing in a new Bush ad that says the Democrat's positions shift "whichever way the wind blows."