Virginia Democrat Richard J. Davis and Republican Paul S. Trible Jr. plan to pump about $600,000 to $700,000 into last-minute media advertising, including a blitz of new negative commercials that seek to ridicule and embarrass the other candidate with his own statements.

Davis campaign officials were ecstatic today about a new radio ad, apparently the roughest yet prepared by their Washington-based media consultant, Robert Squier. It features a tape recording of a stammering Trible groping for words as he answers a question about his "proudest" accomplishment during his three terms as a member of Congress.

"They'll go bananas when they hear this," exulted Davis campaign manager James Carville, who compared the ad to the famous 1980 Roger Mudd interview with Sen. Edward Kennedy (D-Mass.), which was widely credited with damaging the latter's presidential campaign. "It's a stopper."

In the recording made at an Oct. 14 Trible press conference here, the 35-year-old Newport News congressman responds as follows:

"That's a good question. Ah, let me, ah, mention a few of the things I'm proudest of. The, uh, . . . 5-second pause . . . Well, I guess, I guess, a bill that had a marked impact on Virginia and for this country was the legislation that began, the uh, uh, that declared, uh, as a day of national celebration the Yorktown celebration."

Although he hadn't yet heard it, a Trible campaign official called the ad another example of Davis media attacks that have yet to have any impact on voters. "It's the same stuff that hasn't moved them up until now," said Trible strategist Robert Weed. "I think they're sunk."

Trible launched his own wave of negative ads this week, including one 30-second television commercial that hammers at Davis for a glib remark on Social Security.

The Social Security ad was ordered up after Trible campaign polls showed voters turned off by a widely quoted Davis comment, made at an Alexandria nursing home, that was intended to emphasize his commitment to maintaining retirement benefits for the elderly. "I don't care where it the money for Social Security comes from," Davis is quoted saying. "If that's tough luck for the younger generation, that's their tough luck."

Another Trible commercial features flashing still shots of Lt. Gov. Davis, President Carter and Kennedy, whom Davis said he has never met. "It's a clarity ad," said Weed. "If you are in favor of Teddy Kennedy controlling the Senate, then you would be for Dick Davis."

Trible aides said he plans to spend between $400,000 and $528,000 for these and other final-week commercials, while Davis' campaign, which has funnelled more than two-thirds of its entire budget to Squier's media firm, estimated their cost of the final week of ads at between $175,000 and $200,000.

The new wave of negative ads come as private campaign polls showed Davis with a slight but statistically insignificant lead in a race in which both candidates have accused the other of running "negative" ads filled with "distortions" and "lies."

Weed said Trible campaign polls showed a highly unusual mid-October jump in undecided voters from 15 percent to 21 percent. Carville said "Weed's lying," because Davis' campaign polls showed that the undecided group dropped to 13 percent.

Weed had his own interpretation of the supposed jump in undecided voters: "Davis' advertising turned people off to both candidates," he said. "People said this election doesn't make any sense.