Vice President Walter Mondale, fresh from his vigorous campaigning in Illinois, took President Carter's reelection bid into Northern Virginia last night and exhorted Democrats there to stick with the president.

When you've got a good president . . . keep him there working for you," Mondale told about 125 Northern Virginia supporters who staged the region's first fund-raiser for the Carter Mondale team.

The gathering at the Marriott Twin Bridges also gave the vice president a chance to urge Carter supporters to attend this Saturday's crucial Democratic caucuses, where delegates will be elected to the state party convention.

"You've got to go to those mass meetings," said Mondale. "If you're not there, you don't count."

Virginia Democrats will be meeting in counties around the state Saturday to choose delegates to the party convention in Richmond May 16 and 17. Those delegates, in turn, will elect 64 delegates to the national convention in New York this August.

Mondale's campaign appearance in Northern Virginia was particularly important since party officials concede that it will be the only area of the state where Massachusetts Sen. Edward M. Kennedy has any hope of winning a significant number of convention delegates.

Kennedy backers have been particularly active in Arlington and some districts in Fairfax County (where Kennedy makes his home). They are expected to try to pack the mass meetings this weekend on behalf of their candidate.

In the preliminary filing for delegate spots from Fairfax County, for example, a district-by-district survey showed 298 Carter supporters seeking election at the Saturday meetings compared with 226 Kennedy supporters and three uncommitted delegate candidates.

Speaking to a group that included Lt. Gov. Charles S. Robb, Rep. Joseph L. Fisher and more than two dozen Northern Virginia legislators and local government officals, Mondale recited a list of Carter administration accomplishments.

Stressing Carter's in-office experience over Kennedy's mixed successes in Congress, Mondale said that "sitting in the hot seat yourself" is the only way to learn to be president.

The vice president praised Carter's efforts to improve the economy, the nation's energy resources and the problems of the cities.

"Practically every Democratic mayor is supporting President Carter except one -- and Ted's trying to give her back," joked Mondale, referring to Kennedy's avid backing from embattled Chicago Mayor Jane Byrne.