ANNAPOLIS, Maryland (Reuters) - Maryland Governor Martin O'Malley on Monday approved a gradual hike in the state's minimum wage to $10.10 an hour as fellow Democrats seek to make raising the wage an issue ahead of this year's midterm congressional elections.

O'Malley's signature will raise the state's base wage to the new rate by July 2018 in stages from its current level of $7.25 an hour, which is also the federal minimum.

Maryland joins California, Hawaii, Connecticut and the District of Columbia in passing legislation or signing into law increases in the minimum wage.

"Strengthening Maryland's middle class has been the North Star of my administration," O'Malley, who is weighing a run for the presidency, said in a statement.

Democrats headed by President Barack Obama have seized on the issue of raising the base wage of $7.25 as a way of stirring voter enthusiasm heading into mid-term elections in November.

Obama pushed Congress to raise the federal minimum wage to $10.10 but has failed to win the backing of the Republican-controlled House of Representatives. The Democrat-controlled Senate failed last week to muster 60 votes to avoid a filibuster on the issue.

(Reporting by John Clarke; Editing by Ian Simpson and Cynthia Osterman)

[Female fast food worker via Shutterstock]