Maryland Gov. Martin O’Malley (D) on Thursday launched an online petition drive to raise the state’s minimum wage, saying, “We must take action to strengthen and grow the middle class.”

O’Malley’s office has been working on a bill to introduce in next year’s legislative session, but the governor has not said how much he would increase the current minimum wage of $7.25 an hour or how quickly.

In an e-mail sent out by his political action committee on Thursday, O’Malley asked supporters to join what he said will be a “long and difficult effort.”

Bills to raise the minimum wage went nowhere during this year’s 90-day legislative session in Annapolis, in part because of staunch resistance from retailers and other employers. But supporters — including the state’s labor unions — hope the dynamic will be different when lawmakers return in January.

Several Democrats — including all three candidates running next year to succeed O’Malley as governor — have promised to make the issue a priority.

Attorney General Douglas F. Gansler (D) has signed onto a plan that would set a minimum of $10.10 an hour by July 2016.

Another Democrat seeking her party’s nomination, Del. Heather R. Mizeur (D-Montgomery), on Wednesday proposed a plan that would gradually raise the rate to $16.70 an hour by October 2022.

Lt. Gov. Anthony G. Brown (D) has also pledged to push minimum-wage legislation.

Nineteen states and the District mandate a minimum wage higher than the federal minimum of $7.25 an hour.

“Moms and dads are working harder, but falling farther behind,” O’Malley wrote. “As we move Maryland forward out of this recession and into better times, we must make the important choices necessary to expand opportunity and strengthen families — choices that grow the ranks of a diverse middle class.”