Republicans and Democrats are casting wary looks at Patrick J. Buchanan as he considers running for president on the Reform Party ticket, with the GOP worried that he could rob it of its religious-right base and Democrats concerned that Buchanan could become the 2000 counterpart to George C. Wallace, the Alabama governor who broke the partisan loyalties of working-class whites.

Republicans see Buchanan costing their nominee three to four percentage points, enough to give Democrats the White House for another four years in the event of a close contest.

Democrats are less worried, but are gearing up to block a drive by Buchanan to appeal to discontented union workers, angry at trade policies seen as shifting well-paying industrial jobs overseas and south of the border to Mexico.

At the same time, conservative Republicans seeking the presidential nomination are already jockeying to capitalize on Buchanan's expected departure from the field. Both Gary Bauer and Steve Forbes are positioning themselves to be the legitimate heirs to the Buchanan vote and to claim the conservative mantle in the battle against the frontrunner, Texas Gov. George W. Bush.

If in fact Buchanan bolts the GOP, as he has been increasingly indicating, and seeks the Reform Party nomination, he said he will run as a protector of the right to life, an opponent of free-trade policies and a tough-minded advocate of American sovereignty around the world.

In early Republican polls and straw ballots, Buchanan has done poorly, and he argues that the GOP establishment has sold out its conservative base to take moderate stands on abortion, to join with the Clinton administration to back free trade and such military actions as the fight over Kosovo.

Breaking with the Republican Party would be "a tremendous breach, there is no doubt about it," Buchanan said yesterday on NBC's "Today" show. "But I think you've got to put the country above the party." He attacked the Democrats and Republicans: "We've got a one-party system in Washington masquerading as a two-party system."

Fred Steeper, pollster for Bush, contended that "Bush's margin [in the general election] will be large enough to survive Buchanan." In addition, Steeper argued, that "to the extent that Buchanan emphasizes economic nationalism [as opposed to abortion and other social issues], that will draw voters away from [Vice President] Gore more than Bush, from union groups in the 'old' economy."

Former Republican National Committee chairman Richard Bond said that if Buchanan wins the Reform Party nomination--a victory that Bond and others believe will be tough to get--"he will be a royal pain in the butt. The media intelligentsia loves this guy."

Buchanan's initial reception among Reform Party activists has been mixed.

The party's 1996 vice presidential candidate, Pat Choate, indicated that he would be very supportive of Buchanan, and told CNN that Buchanan could get 25 percent of the total vote on Nov. 7, 2000.

Earlier this summer, however, the party was taken over by forces loyal to Minnesota Gov. Jesse Ventura, who seems likely to try to prevent Buchanan from getting the Reform Party nomination--and the $12.6 million in federal money the party is due.

"What appears to be happening is what happens in the worst of politics: Mr. Buchanan sees this pot of money and is trying to get to it. But he brings a set of political agendas that the Reform Party has not, at least in the past, stood for," said Ventura spokesman John Wodele said, referring to Buchanan's social conservatism.

Ventura is "intrigued" by the possibility of a bid by real estate-gambling magnate Donald Trump, Wodele said. "Trump doesn't bring a marriage from another party. He is a business person. He appears to be receptive to free trade rather than a protectionist point of view. He would not have the dependency on PACs and other money restraints that other candidates bring to the election process."

In the meantime, the battle for Buchanan's Republican constituency has begun in earnest. "If Buchanan goes" and the GOP "nominates a moderate candidate [Bush] . . . then there is a potential mass exodus," warned Forbes spokesman Greg Mueller, who said his candidate could stem such an exodus.

"Pat and every conservative should stay and be part of the fight," Bauer said. "I intend to fight to make it a Reagan party now and in the future."

Geoff Garin, a Democratic pollster who conducts surveys for the AFL-CIO and some individual unions, contended that a Buchanan bid would be most damaging to the GOP and to Bush, should he win the nomination. "Bush would have a lot less flexibility on trying to run to the center on abortion," he said.

While Buchanan could appeal to labor Democrats, Garin said, "he has taken enough stands on other things that are repugnant that it makes it much easier to bring those voters back home."

Staff writer Terry M. Neal and assistant polling director Claudia Deane contributed to this report.

CAPTION: Patrick J. Buchanan is indicating that he may seek the Reform Party presidential nomination.