Rep. Barbara J. Mikulski (D-Md.), considered by some Republicans the strongest potential threat to Sen. Charles McC. Mathias (R-Md.) next year, announced yesterday she will not seek the Senate seat, but will run for a third term in the House.

In making the announcement, the Baltimore congresswomen took a slap at President Carter, saying, "It is clear that the president right now would be a liability to any statewide race."

Mikulski, the unsuccessful Democratic nominee against Mathias in 1974, added that "I intend to support the Democratic nominee" for president, which some observers interpreted as an indication she is ready to jump aboard a Kennedy bandwagon if the Massachusetts senator declares his candidacy.

With Mikulski out of the Senate race, the Democratic field appears wide open. State Sen. Victor L. Crawford (D-Montgomery) said yesterday he is "90 percent ready" to seek the nomination. Also frequently mentioned as a candidate is State Sen. Rosalie Abrams (D-Baltimore), the Democratic state chairwoman.

Former State Senate president Steny H. Hoyer (D-Prince George's) said yesterday he is "interested in getting back" into politics, which he left when he lost his party's nomination for governor last fall, but has no plans to run next year.

Hoyer noted that "Mathias would be awfully tough to beat." Crawford, 47, put it this way: "I think I can get the nomination, but the outlook is not very promising for winning the general election."

Mathias, who has announced he will seek a third Senate term after four terms in the House, had no comment on Mikulski's statement.