The troubled campaign of Democratic U.S. presidential contender Martin O'Malley was dealt another blow on Thursday when the struggling candidate failed to qualify for the primary ballot in the political bellwether state of Ohio.

Ohio holds its primary on March 15. O'Malley's campaign submitted 1,175 signatures to the Ohio Secretary of State's Office, but only 772 were deemed valid by the state's boards of elections ahead of the Jan. 5 deadline. That is short of the 1,000 valid signatures necessary to be on the ballot, Joshua Eck, an office spokesman said.

O'Malley's campaign said the former Maryland governor was not giving up on the state.

"While this news is disappointing, we are exploring all of our options, and Governor O'Malley will campaign vigorously in Ohio," spokeswoman Haley Morris told Reuters.

With its 18 of a total 538 electoral votes and its history of choosing presidential winners, Ohio is considered a must-win state for both Democrats and Republicans. The state has voted for the successful candidate in the quadrennial general election since 1964.

O'Malley has failed to gain traction in his challenge to the Democratic front-runner, former Secretary of State Hillary Clinton. The most recent Reuters/Ipsos presidential tracking poll had O'Malley more than 50 percentage points behind Clinton among Democrats nationwide.

Polls also have shown O'Malley badly trailing both Clinton and Senator Bernie Sanders of Vermont in the early-voting states of Iowa and New Hampshire, which hold their contests in early February, raising the distinct possibility that O'Malley's bid may not survive to reach the Ohio primary in March.

(Reporting by James Oliphant; Editing by Alistair Bell and Howard Goller)