CONCORD, N.H., DEC. 14 -- Republican presidential candidate Pat Robertson said today that the Reagan administration is weakening the NATO alliance by signing a treaty to remove medium-range missiles from Europe.

"My major concern is that the United States will undermine our NATO allies," Robertson said. "This has been a long-range goal of the Soviets since shortly after World War II. It looks like they may well be accomplishing that through this treaty."

Robertson's comments came a day after he seemed to be switching gears and backing the treaty. In a news release later in the day, Robertson said reports that he supports the treaty were incorrect.

"If I were in the Senate, I'd have to vote against the treaty as it's now worded," he said.

In another statement, Robertson said, "Although I believe the specifics regarding the missiles appear to be hopeful, I continue to be greatly concerned that without a condition of mutual reduction of conventional arms, we will decouple and undermine our NATO {allies}."

In a televised interview on Sunday, Robertson had called the pact a "pretty good treaty" although he said ratification should be tied to reducing conventional weapons.

At the news conference here, Robertson said that if American nuclear power is removed from NATO, the NATO allies "will be subject to Soviet conventional blackmail."

Robertson said the treaty would be flawed if it is ratified without considering the imbalance of conventional weapons in Europe, but he said he would be "cautiously optimistic" about a treaty that included cuts in conventional weapons.

Robertson also said Soviet leader Mikhail Gorbachev misled the American media during the Washington summit last week.

"He's a master of manipulating our media," Robertson said. "I've never seen anyone as good at it in my life, and the media has gone for it hook, line and sinker."

Earlier, in response to questions here about his lack of political experience, Robertson bristled at his identification as a television evangelist. Since formally becoming a candidate, Robertson has left the Christian Broadcasting Network he headed and resigned his ministry.

"I've never been an evangelist in my life. I'm head of the fifth-largest cable network in America," Robertson said.

On Sunday, Robertson, who had joined three other GOP presidential candidates in opposing the INF Treaty, seemed to be voicing support for it for the first time. Of the GOP candidates, only Vice President Bush has endorsed the treaty. Senate Minority Leader Robert J. Dole (Kan.) has said he will help get it through the Senate but has not decided whether to vote for it.

Robertson, speaking on PBS, said that Reagan has gotten what seems to be a "pretty good treaty." He said that while he does not believe a nuclear war between the United States and the Soviet Union is inevitable, he expects a "clash" between Israel and the Soviets because it is foretold in the Bible.

The candidate stood behind an earlier interpretation of the Bible in which he foresaw Israel being attacked by the Soviets, Iran, Ethiopia and Libya, culminating in a decisive Israeli victory.

One questioner from the audience, citing an often quoted Robertson opinion that only Christians and Jews are qualified to hold office in America, asked if he should "be terrified by that un-American statement."

"I'm not sure it's un-American," said Robertson, explaining that while the Constitution has no religious requirement for officeholders, he believes philosophically that the principles of the founding fathers are best served by Christians and Jews.