Reflecting the power of the conservative movement within the Republican Party, a gathering of 300 GOP liberals and moderates here has become a no man's land for prospective presidential candidates.

Leaders of the Republican Mainstream Committee -- closely tied to the Ripon Society and counting six senators and 11 House members on its board -- complained angrily today that leading presidential prospects were scared away by conservative threats.

"This is the politics of coercion," Rep. Jim Leach (Iowa), the chairman of both the Mainstream group and the Ripon Society, said. "It's the political equivalent of book burning."

Only one presidential prospect, former Delaware governor Pierre S. du Pont IV, has agreed to come, but apparently he aims as much to attack the Mainstream Committee as to seek support.

Aides said that before du Pont addresses the meeting Saturday, he will attend a news conference called by conservatives to denounce the Mainstream Committee's views. He will be joined by Max Hugel, former deputy director of the Central Intelligence Agency; Rep. Henry J. Hyde (R-Ill.) and leaders of a local conservative group, the United Republicans of Illinois.

"We are going to hold a rebuttal," Steven Baer, executive director of the United Republicans, said. "We are going to call into question their presumption, some might say their arrogance, in labeling themselves the mainstream center of the Republican Party. Their smoke screen will be blown away."

Leach, in an interview, said the staffs of at least three potential presidential candidates "were warned that a press conference denouncing all the participants would be held if they arrived." He declined to name these candidates or the persons making the threats.

Baer said he knew of no attempts to "torpedo" the conference and that, if asked, he would have advised prospective candidates to attend and tell the Mainstream Committee "to stick to the straight and narrow." Conservatives sources in Washington, however, said they had attempted to discourage participation in the conference.

The staff of Senate Majority Leader Robert J. Dole (Kan.) expressed some interest in attending, but then backed off. David Keene, a Dole consultant, dismissed the group as "generic Republicans" who don't become Democrats only because "their grandfathers would turn over in their graves."

John Buckley, press secretary to Rep. Jack Kemp (R-N.Y.), said Kemp will be speaking this weekend at the Michigan Conservative Union, "a far more effective use of Congressman Kemp's time."

Last June, Vice President Bush was the keynote speaker at a Ripon Society event that raised $225,000. Ever since, conservatives have criticized him for helping revive that group.

The absence here of all the prospects except du Pont is in sharp contrast to most significant Republican gatherings, where it is hard to keep such politicians away.

The gathering itself is, however, a sharp contrast to most GOP meetings. Among the participants are spokesmen for such groups as People for the American Way, the Sierra Club and Common Cause.

Leach defended the use of "mainstream" to describe his group, saying that the "average Republican supports the Equal Rights Amendment, supports a nuclear freeze . . . is opposed to aid to the contras and is antiapartheid."