En route to his debate showdown in Cleveland with Ronald Reagan, President Carter said here today that his opponent has "flip-flopped" on so many issues that he can't be sure "which Ronald Reagan I am going to face tomorrow evening."

Addressing several thousand people who turned out for an airport rally in this city near the Ohio and Kentucky borders, Carter ran through his familiar campaign litany of what he called Reagan switches on issues such as Social Security, unemployment compensation and aid to New York City.

"The problem that I've had in preparing myself for the debate is which Ronald Reagan I am going to face tomorrow evening on television," Carter said. "Because as the election has progressed and as we get closer and closer to the voting time, he has flip-flopped on almost every conceivable issue depending upon what audience happens to be listening to him at the particular time."

But, the president went on, "We all know which Ronald Reagan will sit in the Oval Office if the Republicans win this election -- the same Ronald Reagan who said just a few weeks ago that he hasn't changed his position in the last 20 years, the Ronald Reagan who worked to kill Medicare, who opposed national health insurance, who rejects a decade of progress in nuclear arms control. . . ."

Carter was accompanied here and later to Cleveland by an unusual ally in a presidential election -- former Israeli defense minister Ezer Weizman. Weizman may not mean much to the voters in this coal-mining corner of West Virginia but his mere presence for a day with the Carter campaign entourage, as White House officials noted, will not go unnoticed by Jewish voters in such key states as New York and Florida.

Asked at the airport if he was endorsing Carter, Weizman said, "Look, the man has done a lot for my country. Do I have to thank him for it or not?"

Reminded of Reagan's strong pro-Israel positions, Weizman all but dismissed the GOP nominee, saying, "I don't know Mr. Reagan."

[In Israel, news that Weizman had joined Carter's entourage was met with disbelief. Foreign Minister Yithak Shamir said in a statement: "It is unbelievable that an Israeli politician, even if he is today just a private citizen, would interfere in the election campaign of a foreign country." Prime Minister Menachem Begin's office refused to comment on Weizman's action.]

The president's brief stop in Huntington was merely a prelude to Tuesday night's nationally televised confrontation with Reagan. From here he flew to Cleveland and went immediately to the Bond Court Hotel, where he is expected to remain while preparing for the debate.

Carter's aides appeared confident about the debate and in good spirits as they headed for Cleveland. Aboard the chartered plane carrying reporters, White House aides passed out what were labeled "Reagan says" fortune cookies which contained "a simple solution in every cookie."

The importance of the debate was reflected by the size of the presidential entourage. Carter was accompanied to Cleveland by virtually his entire political high command, including campaign chairman Robert S. Strauss, pollster Patrick H. Caddel, media adviser Gerald Rafshoon, research director Martin Franks and White House domestic policy adviser Stuart Eizenstat.

But in Cleveland, the president's arrival was not the big news of the day. The headline over the lead story in this afternoon's Cleveland Press was, Barbara Walters on Debate Panel."