“You don’t run just because you think you can win," he said on the "Today" show.
Donald Trump and Ben Carson round out the top three in the new Quinnipiac poll.
The returns show he paid an average tax rate of 36 percent over the years.
Sanders supports a constitutional amendment to undo the 2010 Citizens United decision.
The 2016 hopeful shares his impressions of Mr. Burns and Ned Flanders.
The New Jersey governor made the announcement with a speech at his old high school.
Sanders has surged in polls in New Hampshire.
"He stopped serving as an umpire," the 2016 GOP candidate said.
Bundy made headlines last year and became the face of a land rights standoff in Nevada.
Bush's release pressures Hillary Clinton to release her latest financial figures.