“I am running for president,” Sanders said in an interview with the Associated Press

Sanders will likely make an official public announcement Thursday. The Hill previously reported the senator will hit the launch button through a low-key event or statement, followed by a kick-off event in his home state in early May, and likely trips to the early-voting states to follow.

Sanders has set a noon press conference Thursday on Capitol Hill to discuss his agenda, which will likely include a huge boost in infrastructure and entitlement spending, free college tuition, higher taxes on the wealthy and initiatives meant to keep money out of politics.

The Vermont independent, who has previously embraced the “socialist” label, believes he is well-positioned to be the liberal alternative to Clinton, who is far and away the frontrunner to be the party’s standard-bearer in 2016.

"People should not underestimate me," Sanders told the AP. "I've run outside of the two-party system, defeating Democrats and Republicans, taking on big-money candidates and, you know, I think the message that has resonated in Vermont is a message that can resonate all over this country."

Still, he faces long odds.

According to the latest Fox News poll released last week, Clinton takes 62 percent support, followed by Sen. Elizabeth Warren (D-Mass.), who insists that she’s not running for president, at 12 percent, Vice President Biden at 9 percent and Sanders at 4 percent.

But Sanders has been visiting the carve-out states for months and could make a play for New Hampshire. The Granite State borders Sanders’s home state of Vermont, and the primary voters there are known for having a fierce independent streak.

And many Democrats will be happy to see Clinton get at least a token challenger. They believe it could keep Clinton sharp as she prepares for the general election, and Sanders is the kind of liberal challenger many Democrats believe could keep Clinton from gravitating away from the left.

Sanders has already begun challenging Clinton on policy matters, urging her this month to reject President Obama’s trade push.

Political watchers will be looking to see how aggressively Sanders and other potential challengers go after the former secretary of State, noting there’s a fine line between substantial debate and the kind of political attacks that could do long term damage to the party’s likely nominee.

Sanders has already shown he’s not afraid to throw elbows, saying in an appearance on MSNBC earlier this month that the public is largely unaware of where Clinton stands on myriad policy issues.

“Why don’t you tell me what Hillary Clinton is campaigning on, do you know?” he asked.

“You don’t know and I don’t know and the American people don’t know,” he continued.

“I would hope very much that serious debate on serious issues is what we do in any campaign.”

- Updated at 9:09 p.m.