When Democratic Gov.-elect Charles S. Robb entered his campaign headquarters today, the jangling telephones were witness to his emergence on the national political scene.

Before he could even thank the weary loyalists, Robb had answered calls of congratuations from Republican President Ronald Reagan, who had tried to help defeat him, and former president Jimmy Carter and his wife, Rosalynn. And the final tally had not yet been posted last night when former vice president Walter F. Mondale, a possible Democratic candidate for president in 1984, telephoned his congratulations.

Robb said at a press conference here today that Reagan, who had campaigned for Robb's Republican opponent J. Marshall Coleman, had "communicated informally" before the vote that if Robb broke the pattern of three Republican governors in Virginia, Reagan himself would like to meet with him at the White House.

While basking in the glory of being the first Democrat to be elected governor here in 16 years, Robb said that his wide margin of victory had surprised even him. But he downplayed any national visibility it had brought him, saying only that he is prepared to "play a role" in national Democratic politics.

In victory, Robb -- who did not attack Reagan during the entire campaign -- refused to portray his win as refuting the president's policies. He said that despite Coleman's effort to portray it as such, the race was "not a referendum on the Reagan administration or on anything but Virginia's future."

"I can't explain the margin," Robb said. "It was greater than I anticipated."

Robb's victory is particularly important to Democrats, because he was able to do what their candidates have been unable to do in the past three elections: position himself firmly as a moderate-conservative. He molded an unusual Democratic coalition of blacks, traditional Democrats and suburban residents who in the past had voted Republican.

A bouyant Robb quipped yesterday about the ideological posturing of the race: "I've been pandering to the right long enough," Robb said, after answering several questions from the right side of the room. "Now, I want to pay attention to the left."

Robb's high profile, won from his marriage to the daughter of former president Lyndon B. Johnson, and his handsome, telegenic looks also contributed to his victory, and Robb was asked how important his "star quality" had been in the race.

Grinning and lowering his head in apparent embarrassment, Robb said: "I'm not going to answer that one on two hours' sleep." But he added that Coleman's efforts to link him with the Great Society policies of his late father-in-law "did not appeal to a majority of the electorate."

Robb, who by law can serve only one four-year term, didn't shrink from the national political scene, saying he expects to play a national role not unlike that available to other governors.

"My work is cut out for me here," Robb said. To help him with the transition, Robb announced that he has "another one of those long" documents which he had issued on a wide range of topics during the campaign.

The 50-page report has been in progress for six months, but was kept a secret because it outlines how Robb will handle the Democratic transition of the governor's mansion.

David McCloud, who has been executive assistant to Robb in the lieutenant governor's office, will head the transition team. "He is my only appointment so far," said Robb, who insisted that he has not decided on which of his supporters he will reward with the hundreds of top jobs that will change hands under him.

"If the best qualified individual is already in the job, I won't change just to make a change," Robb said. But with hordes of job-hungry Democrats shut out of top state positions for more than a decade, Robb promised that he would "fulfill my responsibility of party leadership and attempt to build the party" by filling vacancies with qualified loyal partisans.

A Republican source on Capitol Hill said, however, that outgoing Republican Gov. John N. Dalton has vowed to fill every existing vacancy between now and when he leaves office next month.

"The governor is destroyed," the source said of Dalton, who acted as Coleman's campaign chairman and mentor.

Dalton sounded conciliatory at a press conference here today. He said he did not mean to "hurt Chuck's feelings" in strident commercials he made on behalf of Coleman. And Dalton's wife, Eddy, gave Lynda Robb a quickly conducted tour of the governor's mansion this morning.

Republicans lost the governor's mansion last night; at lunch today, they defended one of their favorite Richmond watering holes, W.T. O'Malley's, from a Democratic invasion led by Lynda Robb.

Mrs. Robb and a party of seven entered O'Malley's and found a group of Coleman campaign workers already waiting to be served. "I told her it would be at least 15 or 20 minutes before she could be seated," said Bob Bain, the floor manager at O'Malley's.

When Robb and her friends decided not to wait, their decision was applauded loudly by Republican patrons who had little else to cheer about.

Said Bain, "It made everybody else feel pretty good."