President Carter said today that Ronald Reagan is not to be trusted with the responsibilities of the presidency and that the Republican nominee's election in November would be "a bad thing for our country."

At the end of a campaign swing in which Carter reached back constantly for statements of the Reagan of the past, while the Reagan of the present was in the same state, the president was asked by a television interviewer to assess the impact should the GOP nominee defeat him.

"I think it would be a bad thing for our country if Gov. Reagan should be elected," he replied. "I think a lot of his advisers are very concerned about what he would say in an open and free exchange of ideas with the American people . . .

"I don't know what he would do in the White House," Carter continued. "But his opposition to the SALT II treaty, his opposition to Medicare . . . his call for the injection of American military forces in place after place after place around the world, when diplomatic means ought to solve those problems, indicate to me that he would not be a good president or a good man to trust with the affairs of this nation in the future."

Carter is in the second day of a supposedly new campaign style in which he has said he will attempt to tone down his anti-Reagan rhetoric and discuss "major issues." But while he delivered it in a softspoken manner, his assessment of a Reagan presidency was perhaps the bluntest that he has delivered during the campaign.

Moreover, the president's litany of Reagan's positions including "his opposition to programs like, the minimum wage and unemployment compensation," was typical of a campaign that in many ways has come down to a dispute between the Reagans of past and present.

While the former California governor has sought to moderate many of his positions, the Carter campaign, armed with a fat book of past Reagan statements gleaned mostly from newspapers, has sought to saddle him with his history as a spokesman for conservative causes.

Today, for example, the president's target was the large number of elderly voters in this state, a bedrock of Carter's southern base where Reagan is now conceded the lead. After beginning the day in the state capital of Tallahassee, he flew to this community of retirees and held a "town meeting" in the grand ballroom of the Princess Martha Hotel.

"A Democratic president said we need Medicare to help the senior citizens of our nation to stay in good health," Carter told his elderly audience. "Republicans opposed it. My opponent, Gov. Ronald Reagan, took a nationwide tour as the leader against Medicare. On four occasions at least, he has advocated that Social Security be made voluntary," a proposal the president later said would bring the retirement system "to its knees."

White House officials promptly produced a list of quotes attributed to Reagan, most of them from the 1960s, in which he suggested exploring a voluntary Social Security system and described Medicare as "a foot in the door of a government takeover of all medicine."

Reagan had dropped all references to voluntary Social Security since 1976, and he no longer criticizes Medicare, the minimum wage or unemployment compensation. He has "pledged to strengthen the existing Social Security system.

In the interview with a television station from nearby Tampa, Carter denied the frequent suggestions by Reagan and his advisers that the White House plans some "October surprise" to affect the election.

"There is no way to contrive some sort of false surprise to be sprung on the American people just before the election," he said.

He also said it is his "present plan" to retain both Secretary of State Edmund S. Muskie and national security adviser Zbigniew Brzezinski if he is reelected.

In Tallahassee earlier today, the president signed legislation providing $100 million in relief to state and local governments, the bulk of it earmarked for Florida, for the cost of assimilating refugees from Cuba and elsewhere. In another interview he branded as "ridiculous" Reagan's critcism of administration refugee policy.