Two leading congressional conservatives warned the White House more than two weeks ago that President Reagan's insistence on visiting the German military cemetery at Bitburg was "morally wrong," harming the conservative cause in general and alienating Jewish voters in particular.

That message was conveyed to White House communications director Patrick J. Buchanan by Reps. Newt Gingrich (R-Ga.) and Vin Weber (R-Minn.) in a White House meeting April 15. "We presumed at that moment that the White House was trying to get out of the Bitburg visit," said Weber, who described himself as trying to be helpful. "But we presumed wrong."

The two congressmen, members of the Conservative Opportunity Society, said Buchanan told them that canceling the trip "was not even an option." Buchanan did not return a telephone call yesterday.

"You can't do this," Gingrich told Buchanan, according to Weber. "It's got to be canceled." The president, Weber explained, was "underestimating the firestorm sweeping across the country, destroying the possibility of building a relationship between the Jewish community and the Republican Party."

Then, anticipating Reagan's potential moves, Gingrich argued that the president should not add a concentration camp site to his itinerary, which, Weber said, would "compound the original error." Buchanan disagreed, suggesting instead that it would resolve the problem. Soon afterward, the site of the Bergen-Belsen camp was added to the president's itinerary.

Buchanan was "surprised at how strongly we felt," Weber said. "We felt Bitburg was an obvious political disaster -- the Watergate of symbolism. And it was a moral disaster. It's morally wrong. We were shocked that that was not the feeling at the White House."

In a separate interview, Gingrich said the Bitburg controversy was "a tactical device of the anti-Reagan forces to prevent Jews from seeing who is their natural ally." And yet, Gingrich said he, too, thinks the visit is "immoral."

"We have had a tremendous chance to move the Jewish community," Weber said. "The Jews had provided most of the talent, brainpower and money for the Democratic Party. Many Jews were prepared to vote for Reagan . . . . But the Democrats succeeded [in 1984] in making Jerry Falwell and the religious right a centerpiece issue. A lot of Jewish voters got the uneasy feeling that there is a growing anti-Semitic force within the Republican Party. I don't believe that's true, but I understand that concern. I thought that was a one-time problem. Then comes Bitburg, many candlepower greater than the Falwell issue."

But Weber remains optimistic that Jews can be converted to the conservative cause. "A lot of people don't agree with me -- people in the White House, in the Republican Party generally," he said. "They think Jews are reflexively liberal and unable to break loose from the Democratic Party. Jews are a lower priority for them. You don't have to be anti-Semitic to hold this view. They just think that from the Republican standpoint the Jewish community is hopeless."