"I just visited a polling location in Baltimore and the opposition is working hard to get out the vote," O'Malley wrote in an email from his campaign. "We can still win, but every vote is going to matter."
"Whether we treat every Marylander fairly by protecting them equally under the law could come down to a few votes," the email concludes.
The email suggests that election day activity is making the issue closer than expected.
A Baltimore Sun poll last week claimed the that 47 percent of those who responded would vote against same-sex marriage while 46 percent would vote to legalize it. Another 6 percent either refused to answer or said they were undecided.
Nationally, pollsters agree that the undecideds on this issue tend to break against legalizing same-sex marriage.
On Monday, O'Malley appeared on MSNBC and declared the Baltimore Sun poll "an outlier" and expressed confidence in the legalization of same-sex marriage.
If Maryland legalizes same-sex marriage, it could become the first state to do so through referendum. Nationally, the issue has been defeated in all of the 32 other states on which it has appeared on the ballot.
Follow our live election blog and chat here.