Five prominent conservatives on Friday accused President Bush of abandoning his conservative electoral base and urged him to return to the fold lest he weaken the Republican party and its candidates. If he doesn't, Bush will face an ``almost certain'' challenge from a conservative candidate in the 1992 primaries, one of them warned. Another, Richard A. Viguerie, said Bush ``took a sharp turn to the left'' shortly after his election. ``It is the conservatives in the Republican Party that have produced the money, the workers and the votes all these years,'' said Viguerie, president of the United Conservatives of America. ``Whenever there's a close presidential race and the Republican nominee abandons his conservative voters, they lose,'' Viguerie said. He cited as examples the races of 1948, when New York Gov. Thomas A. Dewey lost to President Truman; 1960, when Richard Nixon lost to John Kennedy and 1976, when President Ford lost to Jimmy Carter. Viguerie and four other leaders of the conservative movement joined together at a news conference to cite issue after issue as grounds for their concern. Topics ranged from trade concessions for China to support for government-financed art that may be considered obscene, but it was clear their biggest complaint was with the president's abandonment of his ``no new taxes'' pledge. Politicians may change their minds but ``when a politician does that on a core defining issue he used to get elected, he sells out not just his followers but he weakens the system,'' said David A. Keene, chairman of the American Conservative Union. Bush has ``dealt a crippling blow to his own credibility,'' Keene said. The conservatives' news conference was timed to coincide with a meeting of the Republican National Committee in Chicago. RNC spokeswoman Leslie Goodman in Chicago said she was unfamiliar with the conservatives' comments and declined to respond. The five conservatives released a letter to party Chairman Lee Atwater protesting that the administration had showed ``that it does not intend to honor the commitments made to the American people in the 1988 presidential campaign.'' The conservative leaders showed some differences among themselves. Viguerie said he thought Bush had sailed under a ``false flag'' of conservatism in 1988, and now was reverting ``to the big-business, country-club kind of Republican he was in the '60s and the '70s.'' Keene, however, said Bush did not sail under any false flags, but rather had been seduced by high poll ratings. The president appears not to know that ``success in the polls has a history of breeding failure,'' he said. Howard Phillips, chairman of the Conservative Caucus, said ``It is too early to tell what any of us is going to do.'' But Phillips said that if Bush ``stays in his present course he will be challenged in 1992, almost certainly in the Republican primaries and certainly in the general election.'' Also participating in the news conference were L. Brent Bozell III, executive director of the Conservative Victory Committee, and John Cregan, president of the U.S. Business and Industrial Council.