IT was really a small thing: The day after Labor Day, Mario Cuomo agreed to meet with the Monitor's breakfast group when he came to Washington a few days later. Yet what a stir it caused. On hearing about the coming event reporters were calling me to say, in one way or another, "It looks like Cuomo is getting ready to run."Then, only a few hours before the event was scheduled, Governor Cuomo's man in Washington, Brad Johnson, called to say that, regretfully, his boss wouldn't be coming for breakfast. The same reporters, on hearing about the cancellation, were quick to assume Cuomo had changed his mind about running. Actually, the New York governor's prime reason for coming to Washington was to attend a congressional meeting on the Democratic domestic agenda. When this meeting fell apart, Cuomo called off his trip. He's simply postponing his meeting with our group until he has another business-connected reason for coming here. What is significant, nonetheless, is the press's sharp interest in every move Cuomo makes these days. Every word he utters is closely examined to see if he's softening his often-expressed position: "I have no intention of running." Every action is similarly scrutinized to see, as one reporter put it, "if he's beginning to look like a presidential candidate." The press has its eye on a potentially big political story. If this long-running saga had a title, it would be "Waiting for Mario." Democratic voters are anxious for Cuomo to run because they know he may be the only candidate who can give George Bush a run for his money. And Democratic public officials at all levels of government want Cuomo running because they think New York's governor, at the head of the ticket, would boost their own prospects for reelection. How long can Cuomo wait? Paul Tsongas, who met with these same reporters the morning before Cuomo had been scheduled to appear, said he thought Cuomo, unlike lesser-known candidates like himself, could wait as late as December to jump into the race. Tsongas says he wants Cuomo in the contest. He sees himself as an under-rated candidate who can best show his potency by knocking off a giant like the New York governor. "Every time I go into New York," says Tsongas, "I call upon Cuomo to run." Tsongas could surprise us all. But most observers think that the nomination is almost Cuomo's for the asking - should he ask. What are Cuomo's prospects were he to run and be nominated? Right now they don't look very good. He'd be up against a highly popular president. And voters tend to keep their presidents for a second term. But the political climate could change. The economy is taking its sweet time to come out of a recession, and it just might not emerge from the slump by November of '92. Indeed, it could take a turn for the worse. That might give a Democratic candidate a decided lift. Also, Cuomo is the one Democrat who could, of himself, give Bush a battle. Put Cuomo in the debating ring with Bush and you might have to give the challenger, with his speaking skills, the edge. It's relevant here to remember the Nixon-Kennedy debate of 1960. Nixon had been the favorite - and sometimes the big favorite - to beat Kennedy until that fall confrontation on TV. And then the challenger - partly with his personality and youthful, vigorous appearance, but also with his performance - showed he could stand up to the formidable Mr. Nixon. That turned the race around. So will he run? One national Democratic leader who is trying to convince Cuomo he should get in, says he now thinks the Governor will soon stop pawing the ground and throw his hat into the ring. But he adds: "Cuomo continues to blow hot and cold. Sometimes he even says he isn't sure he is ready to be president. I sometimes ask myself if he really burns to become president. You've got to burn for the presidency to be willing to put up with all the scrutiny and criticism you get during the political campaign."