Hillary Clinton must really be "feeling the Bern" after last night's campaign rally, when independent Vermont Senator Bernie Sanders addressed another huge crowd, this time more than 7,500 screaming supporters packed into a downtown arena in Portland, Maine.

Sanders is shoring up to be a real threat to the one-time presumed Democratic nominee Clinton, closing the gap in the polls in recent weeks and attracting the largest crowds of any candidate, drawing nearly 10,000 in Wisconsin last week. Monday night, Sanders offered his own opinion on why he's become such a favorite on the campaign trail, reported the Boston Herald.

"The answer, I think, is pretty obvious," Sanders told the crowd at Cross Insurance Arena. "From Maine to California - we have friends from Alaska and Hawaii as well - the American people understand that establishment politics and establishment economics is not working for America."

During the hour-long address, which was streamed live via YouTube on his campaign website, Sanders called "grotesque" and "immoral" income inequality "the great moral issue of our time," reported The Associated Press.

The self-described democratic socialist went on to stress the need to raise the federal minimum wage to $15 per hour, reform the criminal justice system, make college tuition free, invest $1 trillion in rebuilding America's infrastructure and guarantee workers at least two weeks of paid vacation.

He also spoke out against trade agreements supported by presidents of both parties which "have cost us millions of decent paying jobs as corporate America shuts down here and moves to low wage countries abroad."

Sanders told the crowd that he cannot fight alone against the "greed" of Wall Street and corporate America. "When the people stand together, there is nothing we cannot accomplish," he said.

Clinton still leads in practically every poll, but Sanders has been drawing the crowds, and that certainly says something, especially when the Clinton campaign admits it is a bit "worried" about the "serious force" that Sanders is.

"We are worried about him, sure. He will be a serious force for the campaign, and I don't think that will diminish," Clinton Communications Director Jennifer Palmieri said Monday on MSNBC's "Morning Joe."

"It's to be expected that Sanders would do well in a Democratic primary, and he's going to do well in Iowa in the Democratic caucus."

On Friday, Sanders brought out more than 2,500 people to Council Bluffs, Iowa, a record turnout for the 2016 cycle in what will be the first presidential caucus state, reported The Washington Post. Along with the 10,000-strong crowd in Wisconsin, Sanders recently drew more than 5,000 in Denver and some 3,000 in Minneapolis.

"My point is a simple one," he told the Portland crowd. "If we want real change. It's not just electing someone, hopefully me. But it's more than that. Let me tell you right now what no other candidate will tell you. And that is that no one who is elected president of the United States can do it alone. No one in the White House will have the power to take on Wall Street alone, corporate America alone, the billionaire class alone. The only way that change takes place is when we develop that strong grassroots movement, make that political revolution stand together, and then we bring about change."

Watch Sanders' full speech below.