Senate Majority Leader Robert C. Byrd (D-W.Va.) is quietly sounding out Democratic colleagues about whether they would support him for a seventh term as the party's leader in the Senate.

Byrd's soundings -- along with reactivation of a political action committee (PAC) that he has used to aid the reelection campaigns of Democratic senators -- have surprised colleagues who were expecting him to step down when the Senate chooses its leaders for the next Congress at the end of this year.

While Byrd has not disclosed publicly whether he will run again for the top Democratic leadership post, some senators who have talked with him have told colleagues they expect him to.

"Two months ago, I thought he was gone for sure," said a senator who has been watching the maneuvering impartially. "Now it looks like he's back again."

Byrd's moves, reportedly including talks with at least 16 senators during the past couple of weeks, came as Sens. Daniel K. Inouye (Hawaii), J. Bennett Johnston (La.) and George J. Mitchell (Maine) were stepping up their campaigns for the post with the expectation that Byrd would be stepping down.

It is not clear whether any of the three would challenge Byrd if he runs. One senator who is seeking an alternative to Byrd said yesterday that Johnston and Mitchell are not expected to mount such a challenge, focusing pressure on Inouye to do so in the absense of any other likely challenge to Byrd.

Inouye, who is third-ranking in the Democratic leadership as secretary of the party's caucus, has not decided what to do, said the senator, who asked not to be identified. "If Danny doesn't move soon, it will all be over," the senator added.

Senators said Byrd has been buoyed by praise for his response to President Reagan's State of the Union address from colleagues, some of whom previously criticized him as overly preoccupied with parliamentary tactics at the expense of a broader political message. "Some who thought they were trying to make it easier for him to bow out may have wound up unintentionally convincing him to run again," a Byrd critic said.