In the past two presidential campaigns, Patrick J. Buchanan's volatile mix of protectionist nationalism and hard-line social conservatism changed the tone and content of Republican primary fights, leaving in its wake the wounded candidacies of President George Bush and Robert J. Dole.

This time out, Buchanan is hoping to wreak havoc on both major political parties in his run for the nomination of the Reform Party. But many politicians of both parties--and independent analysts--are skeptical that the one-time White House aide and political commentator will make the same kind of splash this time around.

On the eve of 2000, the climate no longer seems ripe for Buchanan's alarmist message: sustained prosperity has overtaken economic fear and doubt; changes in affirmative action and welfare have reduced their potential as conservative rallying points and millions have joined the work force.

At the same time, Buchanan, who thrives on controversy, may have pushed the envelope of credibility to a breaking point. This year, he published a book suggesting the U.S. entry into World War II was a mistake, revived allegations that he is antisemitic and entered into a political marriage of convenience with a Marxist-Leninist faction of the Reform Party to further his prospects of winning the party's nomination.

Those who downgrade Buchanan's threat suggest that he is veering close to making himself ridiculous, alienating veterans, reinforcing his image in some quarters as a bigot, and forming alliances with ideological enemies.

"A guy with a 51 percent negative [rating in surveys] is not someone who makes you shake in your boots," said GOP pollster Bill McInturff.

By his own account, Buchanan is a politician who has earned the animosity of the political and media elite. "They're out to kill my career and kill me as a candidate and to end my influence in American politics. That's exactly right. And you can't walk away from that," he said in an interview.

If he wins the Reform Party nomination, Buchanan wants to expand his traditional base among conservative isolationists, embattled defenders of U.S. sovereignty and veterans of the abortion wars. His aim is to bring in Democratic union workers angered by jobs being shipped overseas.

But Buchanan's dream of becoming a powerful third force in American politics has yet to become a reality. A former White House aide in the Nixon and Reagan administrations who became a celebrated presence on television talk shows, Buchanan surprised friends and enemies in 1992 when he entered the Republican primary contest against President Bush, winning an unexpected 37.4 percent of the vote in New Hampshire. In 1996, Buchanan ran again, winning the New Hampshire primary in a multicandidate field, and remaining a presence through the entire primary period.

This year, Buchanan started out once again as a candidate for the GOP nomination, but his bid never gained the kind of momentum that would make him a serious contender. In the Iowa straw poll last summer, he finished a weak fifth out of eight candidates, and soon after Buchanan dropped out of the GOP and declared his candidacy for the Reform Party nomination.

Now Buchanan has ventured into new territory, abandoning his life-long commitment to the Republican Party for the Reform Party nomination and the $12.6 million in federal money that goes with it.

In some respects, especially on trade, immigration and U.S. sovereignty issues, Buchanan and the Reform Party fit together well. At a more visceral level, Buchanan and many members of the Reform Party also share a deep hostility toward and distrust of the political leadership of both major parties, and of the establishment media.

But in other respects, there is a substantial conflict: Reform Party members tend to be secular and libertarian, hostile to the religious right and in favor of abortion rights. Buchanan is a devout Catholic, views the religious right as an ally and considers abortion an abomination. And the Reform Party's highest elected official, Minnesota Gov. Jesse Ventura, is adamantly opposed to a Buchanan nomination.

Once in the arena, Buchanan likes nothing better than to use hot rhetoric to inflame controversy. Buchanan's version of politics gets him the attention he seeks, but at a cost.

His most recent book, "A Republic, Not an Empire," could well alienate a crucial constituency, military veterans and those who hold strong patriotic values. He suggests in the book that the massive loss of American lives in World War II may not have been necessary, that without U.S. intervention Hitler and Stalin might have destroyed each other's armies.

The book generated an outpouring of what most politicians would consider highly damaging publicity and criticism. For Buchanan, in contrast, the controversy was a welcome opportunity for a fight: "You keep going back at 'em and back at 'em, and back at 'em and let 'em keep hitting you and just keep going back at them. It sort of fades away and you're standing there smiling."

Most recently, Buchanan allowed himself to become subject to ridicule by giving a prominent role in his campaign to Reform Party power broker Lenora Fulani, a black leftist with ties to Louis Farrakhan, whose stands on every issue from gay rights to abortion are diametrically opposed to Buchanan's.

One thing Fulani and Buchanan have in common is that both have been accused of antisemitism. "Pat and I have been demonized as haters. He's the official right-wing hater. I'm the official left-wing hater," Fulani said at a joint news conference, denouncing the charges.

Buchanan has been controversial since he began his career in 1962 as an outspoken conservative on the editorial pages of the St. Louis Globe-Democrat.

He started out as an uncompromising anti-communist and severe critic of the leaders of the civil rights movement, including Martin Luther King Jr. As an aide in the Nixon White House, he led the charge in the attack on the liberal media and the antiwar movement, continuing in that role under President Ronald Reagan.

By the close of the Reagan administration, Buchanan had become increasingly hostile toward the Republican establishment. During the early 1990s, he devised his own version of a right-wing populism in the tradition of George C. Wallace, the Rev. Charles E. Coughlin and the "America First" movement. In 1992, Buchanan began to stress nationalistic themes: American sovereignty, protectionist trade policies and tough immigration restrictions.

In the same year, angered by what he saw as Bush's willingness to support a civil rights "quota" bill and his decision to raise taxes, Buchanan challenged the incumbent president, declaring himself the candidate of displaced and laid-off workers who bore the burden of the decline in traditional manufacturing.

The high point of Buchanan's political career was reached in 1996, when he defeated Dole in the New Hampshire primary. "We almost broke through," said Buchanan. "One more knockdown of Dole and I would have won the nomination. I think the Establishment probably would have opposed me. But you would have had a realignment right there."

In one respect, Buchanan has not changed significantly. He has, in the view of many critics, repeatedly crossed over the border of antisemitism, pushing the boundaries of acceptable political discourse and provoking growing criticism from many civil rights and Jewish leaders. The Anti-Defamation League, for example, has set up a Web site called "Pat Buchanan in His Own Words," listing some of his more controversial comments, including:

* 1977: "Those of us in childhood during the war years were introduced to Hitler only as a caricature--though Hitler was indeed racist and antisemitic to the core, a man who without compunction could commit murder and genocide, he was also an individual of great courage, a soldier's soldier in the Great War. . . . His genius was an intuitive sense of the mushiness, the character flaws, the weakness masquerading as morality that was in the hearts of the statesmen who stood in his path."

* 1983: "The poor homosexuals--they have declared war on nature, and now nature is exacting an awful retribution [AIDS]."

* 1991: "If we had to take a million immigrants in, say Zulus, next year, or Englishmen, and put them in Virginia, which group would be easier to assimilate and would cause less problems for the people of Virginia?"

* 1999: "After World War II, Jewish influence over foreign policy became almost an obsession with American leaders."

The angry responses to such comments have brought out the side of his character he is perhaps most proud of: The boy trained by his father in Chevy Chase to fight, the adolescent who went to parties knowing that he and his buddies would get into a brawl before the night was over.

"We [Catholics] didn't have any girls in our schools so you'd go to their [Protestant] parties and you'd walk into a basement and everybody's there and they're sipping their beer and you know what's going to happen," he said.

Buchanan, before running on his own, was drawn to politicians who were insurgent outsiders opposed by the traditional Wall Street-Rockefeller wing of the Republican Party. "Goldwater wasn't the Establishment, I was with him. Nixon surely was detested by the Establishment. . . . The Establishment mocked Reagan. Reagan triumphed over the Establishment and he succeeded where Nixon failed."

As Buchanan campaigns for his Reform Party candidacy, he is not backing away from the style that has made him a prominent figure through the 1990s. In a recent television appearance, he was asked about one of his critics, New York Times columnist William Safire. "With regard to Mr. Safire," Buchanan said, "I've got to say I represent America first. I represent America only. Mr. Safire, in my judgment, has always put Israel a little bit ahead of his own country."

Safire, who worked with Buchanan in the Nixon White House, is one of a number of conservatives who have called Buchanan antisemitic.

Buchanan, in an interview, clearly enjoys the fray: "We're doing good. We're under attack and I'm proud of the book and I feel I'm fighting on solid ground," he said. "I'm feeling very good in these battles. As long as you're standing there at the end when the other fellow walks away, I think you're not beaten. I think you've won."