Former Maryland Governor Martin O'Malley has been strongly considering taking on Hillary Clinton for the 2016 Democratic presidential nomination, and wasted no time this week calling out the former secretary of state on her same-sex marriage stance.

O'Malley, expected to announce his candidacy in May, took issue with a comment made by Clinton's campaign on Wednesday, when campaign spokesperson Adrienne Elrod said, "Hillary Clinton supports marriage equality and hopes the Supreme Court will come down on the side of same-sex couples being guaranteed that constitutional right," according to the Washington Blade.

While O'Malley didn't specifically mention Clinton in his response, the timing makes it obvious who he was referring to when he tweeted, "The dignity of every person tells us that the right to marry is not a state right, it is a human right" and spoke in this video.

Another issue he has with Clinton's candidacy is that another Clinton or Bush in office could further push the U.S. into oligarchic territory.

"The presidency of the United States is not some crown to be passed between two families. It is an awesome and sacred trust to be earned and exercised on behalf of the American people," O'Malley told ABC's George Stephanopoulos.

O'Malley also served as the mayor of Baltimore, a tenure that was at least partially fictionalized in HBO's hit show "The Wire" as the character Mayor Tommy Carcetti.

Despite earning only 1.2 points in a RealClearPolitics average of polls, compared to Clinton's 59.8 points, O'Malley's campaign hopes that his left-of-Clinton stance and public service record in Maryland, where he served as one of the most liberal governors in the country, will set him apart from other Democratic contenders.

Easing student debt burdens happens to be one of O'Malley's most noteworthy achievements during his two terms as governor. It's an issue he continued to denounce at recent speaking engagements in Iowa and South Carolina, saying that massive students debts continue to prevent young people from buying homes, investing in the economy and starting businesses, reported The Huffington Post.

When he first took office in 2007, Maryland's state college tuition was the eighth highest in the nation. O'Malley took action by freezing state college tuition for four years and ensuring tuition increases remained below 3 percent in the following years. By 2014, the state had fallen to the 27th most expensive.

"It's outrageous that you can buy a home for a lower interest rate than you can get a student loan," O'Malley was quoted as saying in the Washington Post. "We need some very, very serious consideration to making it easier and to find a way to refinance a lot of the debt that kids are under today."

O'Malley hasn't suggested any specific actions regarding forgiveness or refinancing of student loans on the national level, but being that he has been courting supporters of Senator Elizabeth Warren, such an announcement could be right around the corner.

And though he's taken shots at Clinton as of late, he maintains that he's "not against any person," telling reporters, "I'm for what's best for our country. I'm for what's going to make our economy work again for all of us."

He has said he mostly agrees with President Barack Obama's policies on immigration, but, "when refugee children arrive on our doorstop fleeing starvation and death gangs, we don't turn them away - we act like the generous, compassionate people we have always been," O'Malley said in Iowa last week, reported CBS News.

When it comes to foreign policy, he's shied away from committing too strongly to any specific approach, leaving plenty of wiggle room: "I think our most effective foreign policy is a foreign policy of constant engagement around the world, and deploying our considerable diplomatic power, and our economic power, in accordance with our principles," he recently told Salon.

O'Malley has promised to deliver a number of foreign policy speeches in the next few months, "almost certainly on national security and foreign policy," CBS reported.

But the biggest issue O'Malley plans to be tackle is the biggest issue facing Americans: the economy.

"Eliminating tax preferences that benefit the very wealthy, expanding paid leave for working families, and investing in education will all help build a stronger middle class," he wrote in a Facebook post in January.

He'd like to reinstate the Glass-Steagall Act, which prohibited commercial banks from engaing in the investment business with federally insured customer deposits.

"The worst thing we could do is let recent economic progress be undercut by Republican efforts to undo critically important financial reforms, further tilt the tax code towards the wealthy and corporations, and roll back health care coverage for millions of Americans," he wrote. "Instead, we must focus on additional ways to bolster our middle class and working families - by strengthening workers' rights, boosting good -paying clean energy jobs, and reducing the influence of special interests in our political system."

In the meantime, O'Malley plans to meet with voters around the country to increase his recognition. He will be the guest of honor at a reception sponsored by his political action committee, O' Say Can You See PAC, in Los Angeles on April 23, according to the Washington Examiner.