PHILADLELPHIA _ What becomes a Democratic Socialist most?

If you're Bernie Sanders, it's the steady stream of economic populism, Wall Street rage and vows to rebuild the middle class he fire-hosed onto a ballroom full of labor activists here Thursday.

"If we are going to rebuild the American middle class, we have to understand we live in a rigged economy," the Vermont senator told the Pennsylvania AFL-CIO's annual convention.

Sanders' appearance came a day after Democratic front-runner Hillary Clinton made a similar pitch to the trade unionists, and amid escalating tensions between the two campaigns.

At a rally at Temple University on Wednesday, Sanders hammered Clinton, calling her "unqualified" to serve, in retaliation for what he said was her similar claim.

"She has been saying lately that she thinks I am quote-unquote not qualified to be president," Sanders said, according to The Washington Post. "Let me just say in response to Secretary Clinton, I don't believe that she is qualified if she is through her super PAC taking tens of millions of dollars in special-interest money. I don't think you are qualified if you get $15 million through Wall Street for your super PAC."

The accusation provoked an immediate demand from Clinton's campaign for an apology.

"Hillary Clinton did not say Bernie Sanders was 'not qualified.' But he has now -- absurdly -- said it about her. This is a new low," Clinton's spokesman, Brian Fallon, tweeted, according to Politico.

Sanders kept up his attacks during a press conference before his appearance before the labor activists:

"If Secretary Clinton thinks that I just come from a small state of Vermont, 'They are not used to this.' Well, we will get used to it fast," Sanders said, according to The New York Times. "I'm not going to get beaten up. I'm not going to get lied about. We will fight back."

While the candidates' battle unfolded on the web and on cable news, Sanders stayed away from sharp elbows during his 45-minute speech at the Sheraton Downtown.

He twice noted that "Secretary Clinton and I disagree," when it came to such issues as whether to expand Social Security and the expansion of free trade. But the sharp elbows remained holstered.

"We're not going to cut Social Security, we're going to expand Social Security by lifting the cap on taxable income," he said, to applause from the crowd.

In a wide-ranging speech, Sanders touted his support for a $15 an hour minimum wage; a single-payer healthcare system and free public college and university tuition.

He also kept up his drumbeat of attacks on Wall Street and subtly linked Clinton to it.

"I will not leave here and go to a Wall Street fund-raiser," he quipped.

He also returned to familiar criticism of a "corrupt" campaign finance system and again pointed out that he is the only candidate "without a Super-PAC," a claim whose accuracy has been questioned

Staying away from directly attacking Clinton, before a crowd that is broadly supportive of the former secretary of state wasn't the worst of ideas.

With three weeks to go until Pennsylvania's April 26 primary, Sanders narrowly trails Clinton in some public polls. And he is still struggling to close a significant delegate gap.

A Quinnipiac University poll released Wednesday showed Sanders trailing Clinton by 6 percentage points, 50-44 percent.

In hypothetical head-to-heads, Sanders beat two of three Republicans, topping front-runner Donald Trump 48-40 percent and Texas Sen. Ted Cruz 46-38 percent. But Sanders lost to Ohio Gov. John Kasich, a western Pennsylvania native, 46-40 percent.

With 2,383 delegates needed to clinch the nomination, Clinton has 1,749 to Sanders' 1,061. That tally includes both pledged and so-called "super-delegates." Pennsylvania has 210 delegates up for grabs.

On Thursday, Sanders' supporters acknowledged the challenges posed by the electoral math, but remained confident he could close the gap.

"The path is there. He can do it," Paul Betzler, of New Jersey, and a member of the United Autoworkers Union said.

While the union crowd gathered here was mostly pro-Clinton, the Vermont senator has made inroads with labor.

The Communications Workers of America union, which is currently locked in a contract dispute with Verizon, has endorsed him. Sanders rallied CWA workers on Wednesday at an event in Philly's Chinatown.

Samuel Pauling, a Verizon consultant who says he fears he won't be able to cover his children's college and Catholic school tuition in the event of a walkout, said he was gratified by Sanders' show of solidarity.

"It was great to have him there," he said."I don't want to on strike ... I just want a fair deal."

Whether that same kind of affection will extend to Clinton, who, like Trump, comes in the 2016 contest with dramatically high unfavorable ratings, remains to be seen.

Some Sanders supporters have said they'll write-in their candidate's name if he fails to win the party nomination at the Democratic National Convention here in July.

That's a message that's opposite to that of the campaign's. Appearing on MSNBC on Thursday, Sanders' campaign manager Jeff Weaver said "[Sanders] will support the eventual Democratic nominee."

John Dougherty, the business manager of Local 98 of the International Brotherhood of Electrical Workers, and one of the state's most influential labor leaders, dismissed the bluster and predicted Democrats would be unified going into the fall campaign.

"Hillary and Bernie worked together for years in the Senate and they know each other," he said. "And they know Ted Cruz. They know what's at stake. Working people won't entrust their future to a Republican candidate."