WORD HAS it that rumors about President Reagan's mental competence are debilitating his presidency. The truth is that these rumors are debilitating the public's ability to judge their president.

The president's staff has repeatedly tried to diffuse questions about Reagan's health. Such questions, however, have created a recurring drama that helps the president avoid accountability for even his most serious breaches of the public trust. Lies or lack of interest are covered up by his simple admission of memory loss.

The game works like this: the president makes a statement mindboggling in its stupidity or wanders down the misty lane of his memory before the White House press corps and appears senile. Immediately reports of the president's failing health and his incapacity to lead the country surface in the nation's newspapers. The president is kept in hiding just long enough for every American to conjure up a picture of Reagan as an old man lost in his anecdotage.

Then, dramatically, a speech is delivered, a press conference is called or a debate is held. The President seems able to stand up. He seems in control of his saliva. The pundits inevitably notice a renewed sense of "vigor" in the man. For proof of his vitality they point to the one-liner which Reagan always comes up within these situations.

We have witnessed this game many times before. Reagan was down--way down--after he drifted off into an endless and senseless speech at the end of his first debate with Walter Mondale in 1984. Even his friends were worried--the editorial board of the Wall Street Journal, for instance, questioned the president's health. Reagan was kept in seclusion. Americans were given sufficient time to draw their mental pictures of Reagan as an aged grandparent.

But then there was the second debate. Reagan returned appearing unbefuddled and in minimal command of his faculties. Then he cracked a joke about his opponent and issues of his presidential competence were put to rest.

THE LATEST VERSION of this game has been played out over the last few months, with the grand finale occuring last week. The president failed pathetically at his last news conference in November. Unsure of himself, unknowledgable of the facts, befuddled by a press corps which asked him questions, Reagan was down again. Once again, as questions of his health surfaced, the president disappeared.

After the Tower Commission released its report, supporters and critics wondered how the president could allow his policy initatives to "degenerate" and the constitution to be disregarded. These questions of competence, however, soon became entangled with questions of health.

When the President showed last week that he still has the physical stamina to stand up, then, questions of his ability to lead this country dissipated. Even though the president again admitted his utter lack of knowledge about the goings on that resulted in one of the worst foreign policy disasters and breaches of the Constitution in recent memory, journalists told the American people that President Reagan was "presidential." He even managed to crack a one-liner.

ALL OF this is indicative of the apallingly low expectations we have for this president. Convinced of his senility, we wait for Reagan to prove his mental incompetence before the nation. In the process, we disregard more serious questions about his competence. When we begin to equate being "presidential" with being healthy, we degrade the office and make a farce of democracy.

Jesse Jackson knew this three years ago. During a debate of Democratic hopefuls, he said: "I'd rather have Roosevelt in a wheel chair than Reagan on a horse." Jackson knew that President Reagan's health wasn't the standard by which to judge him. His failings as a leader stem from choices he makes knowingly, from nuances of style which he has adopted willingly and from a fascination with vague policy goals which he divorces from fact intentionally.

WE HAVE let the president off the hook because we have never put him on it. After watching four successive presidencies end in failure or disgrace, perhaps it should have been expected that Americans might make a special effort to let Reagan off easy. We have saved Reagan's presidency time and time again by lowering our expectations of him to such an extent that he could not possibly fail.

Still, Reagan has taken advantage of our kindness in the extreme. Unlike the children of an aged parent, we cannot keep making excuses for our elected patriarch. He is accountable, and his health is the least of his problems--as well as ours. To be presidential means to possess a capacity to lead and a willingness to accept responsibility. The more attention paid to the president's health, the less we are able to see his larger failings.