"I'm a foot-stompin' Democrat," was the predominant battle cry last night in the ballroom of the L'Enfant Plaza Hotel, the meeting ground for the annual United States Conference of Mayors.

Many of the mayors assembled were disgruntled Democrats, often shadowed by their image consultants. And many were outspoken on the topics of the New Federalism and Vice President George Bush's unexpected reception last night, which claimed many of their Republican colleagues.

"I'm a Republican," said Theodore DiMauro of Springfield, Mass. "And I'm kind of glad I wasn't invited. I don't want a part of it. I have philosophical differences."

"Bad politics," said Donna Ellman, mayor of Beverly Hills, Calif., of Bush's surprise.

And then there were the new politics amid the milling pin stripes, federal agency employes, industry representatives, stuffed mushrooms and carved-to-order beef.

"It isn't like treating cancer," said Scranton, Pa., Mayor James McNulty of Reagan's new proposals. "This operation means you save the arm, but lose the heart."

"What we've gotten today in meetings with administration spokesmen is a little bit better understanding of the New Federalism," said Nashville Mayor Richard A. Fulton. "And the more we have, the more we do not agree." Most did agree that Reagan's delivery of the State of the Union message was strong and persuasive.

"Reagan is dramatically beguiling," said Janet Gray Hayes, mayor of San Jose, Calif., "but that won't matter as the number of unemployed grows."

"What we have here is a 'sting' operation," said Fulton. "Robert Redford and what's his name were quite a team . . . just like you know who. But this operation is not going to work. Congress is going to stop this one."

Elsewhere the conversation turned to more down-to-earth topics.

"Snow removal," said Springfield, III., Mayor J. Michael Houston.

"Weather is hard to plan around," said Pamela Plumb, mayor of Portland, Maine. "We've both just about reached the end of the line with our budgets for snow."

With the hors d'oeuvres thinning and the crowd planning dinner strategies, a busload of Republicans returned from Bush's reception at the vice presidential residence.

"It was just a very nice, social gathering," said one, tight-lipped.

"I liked it better before they got back," said Seattle's Mayor Charles Royer. "Once the Republicans got back, they started to water the drinks."