Maryland Governor Martin O'Malley speaks at a campaign rally for Lieutenant Governor Anthony Brown, Democratic nominee for Maryland governor, at the University of Maryland in College Park, Maryland October 30, 2014. REUTERS/Jonathan Ernst

WASHINGTON (Reuters) - Former Maryland Governor and possible Democratic presidential candidate Martin O'Malley said on Tuesday he will not seek the U.S. Senate seat being vacated by retiring Senator Barbara Mikulski.

O'Malley, who left office in January and has said he is considering a run for the White House, told reporters in an email he hoped other candidates would step up to represent the mid-Atlantic state, but "I will not be one of them."

The move allows O'Malley, 52, to keep the door open for a potential presidential campaign. He told the Associated Press last month he would decide by spring whether to seek the nomination.

Despite winning two terms as governor in the heavily Democratic State, his future is somewhat complicated by his successor's surprise loss to a Republican in the November election.

O'Malley is popular among Democrats and spent much of the last year actively campaigning for fellow liberals across the country, especially in New Hampshire and Iowa, the first two states with presidential nominating contests.

Representatives for O'Malley have said he plans to visit New Hampshire again in March.

Mikulski, who has represented Maryland in Congress for nearly 40 years, said on Monday she will retire after her current term ends in late 2016.

(Reporting by Susan Heavey and Timothy Ryan; Editing by Jeffrey Benkoe)