Last night, during his State of the Union message at the Capitol, President Reagan took credit for reducing the prime rate to 15 3/4 percent. Down the street, 300 young Washington Democrats thought that was pretty funny.

"I think I'll just go right out and buy a house," snickered Rich Miller.

"Why don't you buy mine, it's been on the market six months," called out Juliet Zucker, who works for the New York State Assembly office here.

The beer flowed sweet and cold, and the quips quick and fast, as Democrats from a group that calls itself the Majority Party gathered to watch the president on wide-screen television at the Capitol Hill Quality Inn. "At times like this it's nice to be among friends," smiled Jonathan Spear, administrative assistant to Rep. William J. Hughes (D-N.J.)

Majority Party is a fund-raising arm of the Democratic Congressional Campaign Committee made up of about 400 Hill aides and various Democrats around town. It costs $50 to join, and the group raised about $4,000 for congressional candidates last night.

"I came tonight because I really love to watch speeches where there's a crowd to hear the humorous comments," said Bill Anawaty, an attorney.

"He's lying," interrupted Brenda Lipson, a co-worker. "He's here looking for a wonderful encounter, a once-in-a-lifetime experience, because of all the lovely ladies."

"That's not true," demurred Anawaty.

"But that's what you told me before," said Lipson.

So it went. They filed in and out throughout the night, flirting, gossiping and criticizing. "There was a time when Democrats were shell-shocked about last year's election," said a former Carter administration aide, Ed Black, who now works with a Washington law firm. "But after watching what's been going on, I think everyone's simply angry now."

As ABC's Sam Donaldson came on the screen and spoke of what he labeled the president's plans to "dismantle" the federal government, one man cried out "Start with OMB, it's right next door. It would make a great art gallery."

At times, you could barely hear the president for the audience one-liners:

This time, however, we have an economic program in place completely different from the artificial quick fixes of the past.

"Yeah, now we're giving away cheese."

Together, we have begun to restore that margin of military safety that ensures peace.

"I'll sleep better now."

If we had not acted as we did, things would be far worse for all Americans than they are today.

"I hope he's playing better in Peoria."

Loudest Boo: When the president talked of the social safety net.

Loudest Laugh: The sight of budget director David Stockman clapping.

And as the president introduced Lenny Skutnik, federal worker and a hero of the Jan. 13 plane crash, James Howarth, Hill aide said: "That guy is probably going to be RIFed next week."