Overflow rallies, a donor file of 300,000 individual contributors, and a rise in early-state polls are evidence that progressive voters across the country are truly feeling the Bern, not just flirting with it, Democratic presidential candidate Bernie Sanders believes.

“This is not a summer fling,” the Vermont senator said during a Hispanic Chamber of Commerce forum Thursday in Washington, in reference to a recent suggestion from Democratic rival Martin O’Malley that Sanders’ rise is simply reflective of the American public’s summer of discontent. “These are issues that are resonating with the American people, saying enough is enough.”

Sanders said his campaign has momentum in the first two early states of Iowa and New Hampshire that is more significant than his national numbers. According to the RCP polling average, Sanders is within 15 points of Hillary Clinton in New Hampshire but trails her by 28 points in Iowa.

“I’m not guaranteeing victory,” Sanders said. “But I think we have a very good chance to win if we develop that grassroots movement.”

The Sanders campaign sought to develop that grassroots effort Wednesday night by organizing house parties across the country for 100,000 people. A New York Times analysis of the attendees found few were concentrated in the two early states.

Sanders, an Independent running in the Democratic primary, told the Chamber of Commerce forum that if he were to lose the nomination, he would not wage a third party bid. “I do not want to be responsible for electing some right-wing Republican as president of the United States,” he said.

In a wide-ranging conversation, Sanders addressed two issues groups have challenged him on recently: race and immigration.

At the progressive Netroots Nation summit earlier this month in Phoenix, both Sanders and O’Malley came under fire for their responses to Black Lives Matter protesters. (Hillary Clinton did not attend the event.)

Asked about race relations Thursday, Sanders said that anyone who does not believe "tragically, that racism is alive and well in America … would be mistaken.” The senator pointed to the recent case of Sandra Bland, who died in jail three days after being taken into custody in an incident that began as a routine traffic stop.

“If that was a white woman, nobody believes that would have happened,” Sanders said, listing sentencing reform, body cameras, and police department reform as policy areas that should be addressed.

Sanders has also come under criticism this week by the immigration reform group FWD.us for comments he made during a Vox interview about easing U.S. border restrictions.

“What right-wing people in this country would love is an open-border policy. Bring in all kinds of people, work for $2 or $3 an hour, that would be great for them. I don't believe in that,” Sanders told Vox. “You think we should open the borders and bring in a lot of low-wage workers, or do you think maybe we should try to get jobs for those kids?”

In a statement, FWD.us President Todd Shulte called the remarks troubling: “He accepts the utterly false premise that our economy is zero-sum, and putting forward the totally debunked notion that immigrants coming to the U.S. are taking jobs and hurting Americans – specifically young people, Latinos, and African-Americans.”

When asked by a reporter about the statement, Sanders said the question at issue was whether to completely open the border so that anyone could enter the United States. Doing so, he said, would undoubtedly lead to lower wages in this country.

“When you have 36 percent of Hispanic kids in this country who can’t find jobs, and you bring a lot of unskilled workers into this country, what do you think happens to the 36 percent of them who are today unemployed? Or 51 percent of African-American kids?” Sanders said in response. “I don’t think there’s any presidential candidate, none, who thinks we should open up the borders.”