Speaking at the Newseum in Washington on Thursday, Sanders said that if he ran a third-party campaign, it would draw support away from the Democratic nominee, potentially handing Republicans the White House.

“I would not want to be responsible for electing some right-wing Republican president,” Sanders said.

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However, Sanders, an Independent and self-described socialist who caucuses with Democrats in the Senate, acknowledged that it was something he considered.

“As I was contemplating what I’d do, one decision I had to make was, there were a lot of people telling me to run as an Independent,” Sanders said. “They said the Republican Party is an extreme right-wing party, and the Democratic Party is too conservative, too cozied-up to big money … and that I should run outside of the two party system.”

“I thought about it,” Sanders said. “But I reached the decision … that the only way at this particular moment in history that we could run an effective campaign was within the Democratic primary and caucus system.”

Sanders said his focus now is solely on winning the Democratic nomination.

He’s emerged as the biggest threat to Hillary Clinton and has galvanized support from grassroots liberals who have turned out in the thousands to hear him speak.

“We have the momentum in early states like New Hampshire and Iowa,” Sanders said. “I think we have a very good chance to win if we develop that grassroots movement.”

Clinton is still the prohibitive favorite to win the nomination, and she leads national polls by huge margins over Sanders. But in the early-voting state of New Hampshire, Sanders has closed to within 10 points of Clinton, according to the most recent NBC News-Marist poll.

To build on those gains, Sanders will have to improve his standing in the polls with minority voters, who overwhelmingly support Clinton right now.

At the forum Thursday, which was organized by the U.S. Hispanic Chamber of Commerce, Sanders acknowledged that many minority voters were hearing about him for the first time.

“I have to be honest, I come from a state that’s 95 percent white, that’s the reality,” Sanders said.

But he argued that his message of income inequality, universal healthcare, free college tuition and a pathway to citizenship for those in the country illegally should be one that resonates with Hispanic voters.

“We’re making a bit of progress, but you’re going to see us doing more outreach to the Latino community,” Sanders said. “I think our economic agenda that says we have to reach out to low-income working families and give them dignity, jobs and decent income, that will be a proposal I think that many Hispanic families will respond to.”

Sanders focused heavily on the issue of race, saying he believes racism is still prevalent in the U.S. today.

He said the nation must overhaul its drug laws and address how police departments are training their officers to use force. He pointed to Sandra Bland, the black woman who died in a Texas jail after getting pulled over for a minor traffic violation.

“If that was a white woman, nobody believes that would have happened,” Sanders said.

Sanders sidestepped a question on whether anyone running for the Republican presidential nomination was a racist. But he denounced remarks made by Donald Trump, in which the GOP candidate described illegal immigrants as "rapists" and other criminals.

“The statements Donald trump made were clearly outrageous,” Sanders said. “It troubles me very much that a candidate for the presidency would stoop to that level.”