President Reagan is unwilling to accept reports of his political demise. He has been sending all sorts of signals that he intends to be a presence in the 1988 presidential election campaign, like it or not.

Some will not like it. The conventional wisdom among Reagan's adversaries is that he is already irrelevant to the political process. Even some staunch Reagan supporters believe that the time has come to look to other leaders.

But Reagan is not ready to abandon the political stage and leave a conflicted legacy to history. He views the conventional wisdom as wishful thinking. His strategists say Reagan is determined to be a factor in 1988.

"I don't think that in 1988 Ronald Reagan can determine the future, but he can define the present," said White House political adviser Frank Donatelli. "In a strategic sense, he can defend his record and point to his accomplishments."

As framed through the White House lens, these accomplishments are "peace and prosperity," the staple issues of a winning presidential campaign. They require the cooperation of the economy, now in its 59th consecutive month of expansion, and of the Soviets, at least until Reagan and Mikhail Gorbachev meet in Washington this fall to sign a treaty that would scrap intermediate-range nuclear missiles.

Reagan's advisers have to do a certain amount of tea-leaf reading to determine that the president has the next election in mind. Even before he entered the isolation booth of the Oval Office, Reagan did not spend off-hours discussing political strategy. He is totally uninterested in the nuts-and-bolts aspects of the process and maintains the useful fiction, which he probably believes, that he is not really a politician.

But even a weakened Reagan understands how to seize the political agenda, which is what has made him so formidable for so long. Last week, the president signaled in what he said and what he did that he understands the importance of staking out the "peace and prosperity" issues for the Republican Party.

The most important signal came in a bizarre ceremony in the White House Rose Garden where Reagan denounced and then signed a limp-wristed deficit-reduction bill that Defense Secretary Caspar W. Weinberger had urged him to veto. The measure could mean an automatic military budget cut of $10 billion or more, giving Weinberger the kind of budget battleground on which he usually prevails. He didn't this time because Republican politicians on Capitol Hill and Reagan's staff convinced the president that the GOP could not afford White House opposition to deficit reduction.

The other signal came during an interview in which Reagan steadfastly defended the arms treaty that he is about to conclude with Gorbachev against the observations of the interviewer that it would leave the Soviets with an overwhelming conventional-arms advantage in Europe. The treaty, favored by more than four-fifths of those surveyed by the White House, translates into favorable television footage and the "peace issue" when Reagan meets Gorbachev. It could produce even bigger political dividends if it leads to another summit in Moscow during the election year.

Reagan's advisers anticipate that he will be called on to play a significant role in the 1988 campaign, especially if the nominee is Vice President Bush. They say Reagan, who intends to stay neutral until the GOP nominee is known, privately prefers Bush and expects him to win the nomination. They do not believe that Bush will repeat the 1960 mistake of Vice President Richard M. Nixon, who let President Dwight D. Eisenhower stay on the sidelines just long enough for John F. Kennedy to become president.

Reagan knows that he cannot transfer his own popularity -- or his own deficiencies, for that matter -- to Bush or any other candidate. What he can do is "defend the present," which he hopes will be one of peace and prosperity. These were the political markers that he laid down last week.

Reaganisms of the Week: At the Rose Garden ceremony last Tuesday, the president said: "To those who say we must weaken America's defenses, they're nuts. To those who say we must raise the tax burden on the American people, they, too, are nuts."

Asked at a picture-taking ceremony the next day if the late CIA director William J. Casey had engaged in covert activities without his knowledge, the president replied: "Not that I know of."