Virginia Gov. Charles S. Robb yesterday disavowed plans to run for national office, but said he will remain active in national Democratic Party politics after his term ends next month and will step up his study of defense and foreign policy issues.

"It's an area I plan to -- simply as a matter of personal interest -- spend a little more time in after I have put down the responsibilities of . . . state government," Robb told a breakfast meeting of national political reporters at the Sheraton Carlton Hotel.

Robb, who described himself as "pro-defense," repeatedly insisted he is not attempting to lay any groundwork to run for president or be available for the vice presidential nomination on the Democratic ticket in 1988, but he again refused to rule out such considerations.

"I don't absolutely rule anything out in politics," said Robb, the son-in-law of former President Johnson. Robb turned aside some questions on foreign policy and defense because he said he wasn't prepared to discuss them in detail.

"I would like to become a more avid reader of some of these things and really kind of make my own judgments," Robb said. Robb said some Democrats have suffered nationally because they have appeared weak on national security issues.

In response to questions, Robb said he generally supports "the basic thrust" of President Reagan's Central American policy but said he has concerns about funding the controversial MX missile because of questions about how it could be protected from enemy attack.

Robb, 46, who under Virginia law cannot succeed himself, is a political moderate and is credited with breaking a 12-year GOP hold on Virginia's highest elected office and building up the state Democratic Party that swept all three statewide offices last month.

Some national Democrats have begun promoting Robb for the party's 1988 ticket and this week Republican National Committee Chairman Frank Farenkopf suggested Robb's success in Virginia could spell problems for the GOP.

In an interview with Washington Post reporters and editors yesterday, New Jersey Republican Gov. Thomas Kean, who was overwhelmingly reelected this year, singled out Robb when asked to name potential Democratic candidates.

Robb, who plans to move back to his McLean home and practice law, has kept up a hectic schedule of political appearances outside Virginia and will join a major law firm next month that will allow him free time to travel and support candidates in the 1986 elections.

Robb was a founder of the Democratic Leadership Council, a group of elected officials whose stated goal is to move their national party toward the political center.

Robb repeatedly has said he has "no present plans" to seek national office and said that was not a suggestion he might change his mind later. However, he suggested he might change his mind "if I am so bored [with private life] or the itch gets so bad."

Robb said voters "see a whole new tier of potential national leadership emerging" within the Democratic Party. Among potential presidential candidates, Robb said he would like to see Sen. Sam Nunn (Ga.) "take a more active role [but] I don't think he's going to do it." Robb also favorably mentioned Rep. Richard Gephardt (Mo.), Arizona Gov. Bruce Babbitt and Sen. Joseph R. Biden Jr. (Del.).

He said Sen. Edward M. Kennedy (Mass.) would have a difficult race in the South. "Philosophically, he and I are not normally mentioned in the same breath."

Robb said Kennedy and other more liberal Democrats have recently indicated a move toward the political center but that Kennedy has "a good deal of work to do."