"Excited to join @corybooker on the campaign trail in New Jersey this Thursday," he tweeted.
According to PolitickerNJ, O'Malley will make stops on his own in Trenton and Paterson and appear with Booker in Newark, N.J.
He's previously helped another New Jersey Democrat, gubernatorial candidate Barbara Buono, to raise cash, lending his name to a fundraising email sent out by her campaign last month.
Booker remains the front-runner in the New Jersey special Senate election, and will likely nab the Democratic nomination in next week's primary. Democrats are expected to win the seat in the general.
O'Malley has begun to dip a toe into 2014 campaigns elsewhere, contributing through his O’ Say Can You See PAC to Iowa Rep. Bruce Braley (D), who's running for Senate there, and New Hampshire Sen. Jeanne Shaheen (D), up for reelection next year. His dabbles in campaigns in early primary states have fueled rumors that he's preparing a presidential bid.
Booker, too, is considered a potential 2016 presidential or vice presidential contender, though he said recently he has no plans to run on a presidential ticket and is instead focused on the upcoming special.