PORTLAND, Maine — Bernie-mania swept through here last night, as more than 7,500 screaming, sign-waving supporters packed a downtown arena — many lining up hours early — for surging Vermont U.S. Sen. Bernie Sanders.

The outpouring came as a Bay State Democratic congressman warned one-time sure-shot Hillary Clinton has an “excitement problem” in the Democratic race for the White House.

“In case you didn’t notice, this is a big turnout,” Sanders told the cheering 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.”

The rally started 20 minutes late so the people stuck in line outside could filter in, and nearly every seat was filled. Some supporters waved homemade signs, including one that read “#Bern Baby Bern” and “Feel the Bern.”

“We love you,” one woman shouted toward the end of Sanders’ speech.

Thousands more watched a YouTube live stream of the rally on the campaign’s website.

“Hillary is not bad. It’s just, we all know she is having an excitement problem. We all know that. It’s not a secret,”

U.S. Rep. Michael E. Capuano said in a Boston Herald Radio interview yesterday. “Everybody wants to fall in love with the perfect candidate. Everybody wants that. So do I. Who doesn’t?”

Capuano said it’s still a “longshot” that Sanders could knock off Clinton, but admitted the Vermont senator’s popularity is hard to ignore.

“Especially if you share a fair number of his values, he’s a pretty exciting guy,” said Capuano. “He’s a street fighter. He’s been a real mayor. He’s always been on the outside looking in as far as establishment politics goes. So I think on some levels, he’s lighting it up pretty good.”

Sanders drew nearly 10,000 people at a rally in Wisconsin last week and polls have shown him closing the gap with Clinton, once seen as the automatic Democratic nominee.

The Vermont senator’s supporters, who lined up as early as 3 1⁄2 hours ahead of time, touted his straight-shooting middle-class pitch and labeled Clinton as out of touch.

“I don’t think she’s my voice anymore — I was hoping she would be,” said Rich Stafford of Salem, Mass. “She’s just too tied into corporate America, too tied in with the banking system, millionaires and billionaires. Her husband’s getting $500,000 to give a speech. How many Americans are in that category?”

John Wibby of South Portland said even if Clinton starts talking about the progressive issues Sanders has been championing, he can’t trust her.

“She seems really not that genuine to me,” said Wibby. “She seems to flip-flop a lot on the issues.”

But members of the Bay State delegation backing Clinton dismissed she’s struggling to get voters — or activists or donors. U.S. Rep. Niki Tsongas pointed to a packed Provincetown fundraiser she attended last month for the former first lady.

U.S. Rep. Stephen F. Lynch, a Clinton backer in the 2008 race as well, warned Sanders has no chance of winning a general election.

“If Bernie Sanders is our nominee, we will have a Republican president. And that is not exciting to me,” said the South Boston Democrat, who said he’s been asked by Clinton’s people to help campaign in Iowa, as he did for Al Gore and John Kerry in past presidential elections. “Whenever they need me, I will make myself available.”