Independent Vermont Senator Bernie Sanders announced Thursday he's running for president.

“After a year of travel, discussion and dialogue, I have decided to be a candidate for the Democratic nomination for president,” Sanders said in an email to supporters and media.

On Wednesday, he told the AP that people should not “underestimate” him.

“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.”

Sanders has previously said that his campaign would focus on trade, income inequality and health care. He is only the second declared candidate after Hillary Clinton for the Democrats' presidential nomination.

Earlier this week, Sanders encouraged the Supreme Court to strike down state bans on gay marriage.

“Of course all citizens deserve equal rights,” Sanders said. “It's time for the Supreme Court to catch up to the American people and legalize gay marriage.”

As a House member in 1996, Sanders voted against passage of the Defense of Marriage Act (DOMA). The Supreme Court in 2013 struck down a key provision of DOMA, which led to the federal government recognizing the marriages of gay couples.

Clinton endorsed marriage equality in 2013.