In a move aimed at undercutting David Duke's chances in Saturday's Senate contest in Louisiana, Ben Bagert, the Republican-endorsed candidate in the open primary, dropped out of the race yesterday and said he would vote for the Democratic incumbent, Sen. J. Bennett Johnston. Bagert's decision was seen as virtually assuring Johnston's reelection to a fourth term.

Bagert, a state senator from New Orleans, had remained in single digits in the polls. He was viewed by Republican officials in Washington and Louisiana as a sure loser in the three-way race whose meager support on Election Day would only help Duke survive the primary and allow the former Ku Klux Klan leader to move into a November runoff with Johnston.

With Bagert removed from the race, Johnston is expected to attract more than 50 percent of the vote in Saturday's head-to-head matchup with Duke and win reelection outright.

"I think it's over," said University of New Orleans political scientist Susan Howell. "It certainly makes it easier on Johnston. I suspect most of Bagert's vote will go to Johnston, because his support was almost exclusively among upscale white Republicans. Duke has little appeal to them; they're the one group he hasn't been able to break through to yet."

National Republicans had become increasingly embarrassed and troubled by the possibility that Duke, elected to the Louisiana legislature as a Republican last year, would campaign this fall as the GOP standard-bearer. Although many of his positions on issues such as affirmative action, welfare and taxes are within the mainstream of conservative ideology, his past in the Klan, his current leadership of the National Association for the Advancement of White People, his ties to the American Nazi Party, his past expressions of admiration for Hitler and his overt race-baiting made Duke unpalatable to the party hierarchy.

Viewing Johnston as the only alternative, eight Republican senators endorsed the Democratic incumbent this week in Washington, and GOP party leaders moved strongly behind the scenes to encourage Bagert to withdraw. Bagert's campaign had been in disarray for weeks, but internal polls conducted this week persuaded him to call it quits.

"It became more apparent to me that instead of forcing a runoff between myself and Bennett Johnston, I might very well be forcing a runoff between someone else and Bennett Johnston," Bagert said yesterday.

"We didn't push him out," said a senior Republican Party official in Washington who asked not to be identified. "But we laid out the options when it became obvious that he was not going to get the momentum to beat Duke."

The party invested heavily early in the Bagert campaign, but cut off the flow of funds to him when it became clear that his candidacy was not picking up steam, the official said.

"We are against Duke, no matter who he runs against, when or where," the official said. "Our top priority was to win the seat, but once it became clear there was not the potential, our top priority was to get him {Duke} off the playing field, to beat him on Saturday."

Duke has been drawing large and boisterous crowds in every corner of the state and has mounted enough of a media campaign to concern Johnston. Pollsters expected Duke to get 30 to 40 percent of the vote Saturday, which in a three-way race might have carried him into a November runoff against Johnston. But according to Howell and other Louisiana pollsters, 40 percent is "the lid" of Duke's support, making it difficult if not impossible for him to win a head-to-head statewide race.

Staff writer Thomas B. Edsall contributed to this report.