Arlington Del. Warren G. Stambaugh was spending a rare quiet eveing at home the other night when he answered his phone and go a surprise order to "stand by" for the president.

"I thought it was a joke," said the astonished Democratic state legislator who was an ardent supporter of Carter's chief party rival, Massachusetts Sen. Edward M. Kennedy.

"He said he hoped I'd support him at the mass meetings," said Stambaugh, one of hundreds of Virginia Democrats expected to attend city and county caucuses on Saturday to elect delegates to the state party convention.

It helped. Stambaugh will be running Saturday as a delegate for Carter.

Direct action such as phone calls from Carter, coupled with Kennedy's clobbering in the Tuesday Illinois primary, are fast eroding what Kennedy supporters had hoped would be a strong showing this weekend for their candidate.

State party officials yesterday reported that only Northern Virginia and the Norfolk-Virginia Beach area had shown any significant interest in Kennedy, in terms of the numbers of candidates filing for election as delegates.

Preliminary reports from around the state showed 1,334 delegate for Carter compared with 339 for Kennedy, six delegate candidates for California Gov. Jerry Brown and 49 uncommitted.

The state convention will choose 64 delegates to the Democratic National Convention in New York this August.

In Northern Virginia, where Kennedy was given his best chance to capture delegates, the mass meetings at 17 locations in the 8th and 10th congressional districts are still expected to attract a sizeable number of Kennedy backers in Arlington and in the Centreville and Dranesville sections of Fairfax County. (Both Kennedy and his sister-in-law, Ethel Kennedy, live in Fairfax County.)

Stambaugh, who was on the verge of running as a convention delegate pledged to Kennedy, said that "it's a very impressive thing to get a phone call from the president of the United States."

"It wasn't really the phone call" that changed his mind, said Stambaugh, but the six-minute chat with Carter didn't hurt. Though he waited a week after the conversation before committing himself, Stambaugh said he felt more confortable about his switch after deciding that the Carter-Kennedy contests had raised some important campaign issues.

"I think Ted Kennedy has been a very good senator, and he cares about the things I care about," Stambaugh said. "But when a president of the United States decides he ought to be a politician and starts calling lowly members of the General Assembly, you've got to start thinking that he's paying attention to the problems."

And, calling himself a politician too, Stambaugh said he also likes "to win elections, and I don't like the idea of a Ronald Reagan as president."

The attitude is exactly what the Carter people in Virginia are counting on Saturday, although many concede that they have detected little enthusiasm among those who have decided to back the incumbent president.

At a Carter-Mondale fund raiser in Northern Virginia Tuesday night, for example, one Fairfax County Democrat cautiously fetched his favorite campaign button from the inside of his jacket. It read: "Probably Carter Despite Everything."

Kennedy's supporters in Northern Virginia still hope to tap some of this lack of enthusiasm for the party's front-runner and turn it to their advantage.

"It comes down to who do you dislike the most," said Ira Lechner, a former Arlington legislator who is actively backing Kennedy. "We present the only other game in town."

Lechner said he hoped Virginia Democrats would use the Saturday mass meetings to register a protest about the way Carter has been handling the economy and foreign policy.

Calling the political situation fluid, the Arlington attorney said a worsening of inflation or concern over re-instituting draft registration could swing voters away from the president.

"It's amazing to me that so many Democrats continue to be so complacent," Lechner said. "If Jimmy Carter was a Republican, 98 percent of those complacent ones would be screaming for his head."

Bob Revels, a Kennedy coordinator in the Lee District of Fairfax County, acknowledged yesterday that the Illinois results "won't be any help, that's for sure" in efforts to attract a big Kennedy turnout. But he denied assertions by Carter supporters that the steam had gone out of the Kennedy movement.

"Just as fast as it changed for the better for Carter, it can change the other way," Revels said. "Once the public can focus on the fact that nothing has changed domestically while attention has been diverted on foreign policy, I think we'll see that change."

Carter supporters, including Loudoun County coordinator Leland Mahan, say that is wishful thinking.

"If our people turn out, it will be overwhelming for Carter," said Mahan, who reported that some of his county's uncommitted Democrats have recently started jumping on the president's bandwagon.

Fred Berghoefer, who chairs the Arlington Democratic Committee, was a bit more cautious, warning that Kennedy's support is coming from the left element of the party -- traditionally its most active.

But he pointed to Stambaugh's defection as a sign that Carter and his campaign organization are doing everything possible to turn out their supporters in Virginia and elsewhere.

Dottie Schick, Berghoefer's counterpart in Fairfax County, said Saturday's results might ultimately hinge "on a more personal level," such as the ability of the individual delegate candidates to get their own supporters to the mass meetings.