Yesterday morning, Tip O'Neill was on the phone to the president, and all day long he was deep in the political fray, but last night the speaker of the House abandoned reality's harsher notes to sing to his bride of 40 years.

I'll be with you in apple blossom time," he sang to his wife, Millie, just as he has every year on their anniversary. He sang in a voice clear and true and tender and he sang with all the sweet sentiment an Irishman can bring to a song, this big rough-hewn politician who knows how to go the distance. The only difference was that this time, listening with the rapt look only romance can elicit, were the 200 Democrats who had plunked down $1,000 a plate to rally around the Democrats' tattered banner at the 1981 Democratic Congressional dinner.

"I'd sure hate to be those Republicans," said Robert S. Strauss, the master of ceremonies for the evening, from the microphone as the o'Neills went down to the dance floor while blue and white balloons floated down from the ceiling to pop under high heels. "He's a hard act to follow." O'Neill was certainly the only act to wrap the beleaguered Democrats in a warm glow of camaraderie and good feeling: His arrival brought them to their feet in a standing ovation at a dinner that, for all the upbeat patter and brave visions of the future, seemed subdued in mood.

O'Neill wasn't saying last night what the president and he had talked about. "That's a private matter," he said judiciously, referring to the phone call that he received from Reagan after the president accused him of "sheer demagoguery" at his press conference Tuesday. But O'Neill did say that he was feeling good last night "because we're on the way back. Everything the president is saying is making people more conscious of what is actually in his program, what the budget cuts really are, and that the tax cuts are geared to the rich."

A couple of tables down at the dinner, where no one seemed to keep his seat for longer than five minutes before springing up to work the room again, Strauss was warmly defending the speaker against those who have wondered whether O'Neill hasn't in fact had his last hurrah. "You never look perfect when you don't have the votes," Strauss said. "Long after Tip O'Neill's critics are gone, he'll be around to get those votes."

Strauss was pleased, he said, to see at the dinner "an awful lot of new people here, the kind who usually just send a check, but decided that now it was time to show that they're Democrats." There was one Democrat who didn't show up, and Strauss volunteered why that was. "I talked to Jimmy about it and he said that whatever he thought of Reagan's programs, he thought it was better to show a little class and stay out of town and let the new president have his day."

Strauss allowed as how there was a lot of talk about 1984 running around the tables, but he for one thought it was too early to be talking about an election so far away. He said this very sincerely and with what looked to be a successful attempt at a straight face.

Making the rounds of the tables with the coy look of perennial prom queens were Sen. Edward Kennedy (D-Mass.) and former vice president Walter Mondale. Occasionally the two would run into each other at the same table and there would be hearty smiles and a slice of conversation all around as the photographers clicked and the reporters descended. At times, it made for an interesting dance, what the politicians trying to smile and sidle away at the same time they were dodging frantic waiters and trying not to trip over the Secret Service.

Mondale was going off to Europe soon to study NATO issues, which he said are the sort of thing he likes to chew over with Rep. Tim Wirth (D-Colo.), who was standing there by his side. Well, was Mondale going to run for president in 1984 or not?" "Gee, I haven't thought about that until just now," he said, smiling brightly through a tan attained while fishing in Minnesota.

The dinner was expected to gross $1.6 million, "by far the most successful we've had," said Rep. Tony Coelho (D-Calif.), chairman of the Democratic Congressional Campaign Committee. "People have something to rally around now." Something, maybe, but not someone, no heir apparent ready to lead the party forward in 1984. Coelho, however, even managed to be upbeat about that. "Maybe without stars we're more successful," he said. "There's a general perception that what we need now is to get out act together."