James Hill for The New York Times

COUNCIL BLUFFS, Iowa — Criticizing Mitt Romney in the first presidential debate, his voice now indignant, now deeply sarcastic, Vice President Joseph R. Biden Jr. delivered the kind of impassioned response to the Republican nominee on Thursday that many Democrats said they wished they had heard from President Obama.

Mr. Biden addressed some 600 supporters here after speaking briefly to reporters, who asked for his impressions of Mr. Romney’s performance.

“Last night we found out he doesn’t have a $5 trillion tax cut,’’ Mr. Biden said. “I guess he outsourced that to China or something.’’

He defended Mr. Obama’s low affect and refusal to engage Mr. Romney more aggressively. “I think the president did well; he was presidential,’’ he said. “As time goes on, meaning days, it’s going to become pretty clear that Governor Romney has either changed a number of his positions or didn’t remember some of his positions. I think at the end of the day we have two more debates coming up, or the president does, and I feel very good about it.’’

Anticipating his own debate with Representative Paul D. Ryan one week away on Oct. 11, Mr. Biden said he had been studying Mr. Ryan’s positions to avoid making any factual errors describing them. “All debates are tough. But I’m looking forward to it, I really am,’’ he said.

At his rally, Mr. Biden delivered scorching responses to positions Mr. Romney espoused on Wednesday night on Medicare, deficit reduction and education. He accused Mr. Romney, often sarcastically, of backing away from earlier positions. He said he was “stunned” to hear Mr. Romney say he planned no cuts to education.

“Yo!’’ Mr. Biden said, pretending to address Mr. Romney. “That came as a real surprise to your Republican colleagues in the House, man.’’

(The House-passed budget written by Mr. Ryan calls for deep cuts to social programs without specifying amounts. The White House has calculated that if the cuts are applied equally across all programs, they will chop about 20 percent from each one.)

At another point, Mr. Biden dropped his voice to a deep baritone meant to mock Mr. Romney, saying, “I don’t have a plan to cut $5 trillion.’’

He laid out the Democrats’ version of how the various tax cuts Mr. Romney proposes – including a 20 percent reduction in income taxes, eliminating inheritance taxes and extending all the Bush-era tax cuts – add up to $5 trillion over a decade.

He contrasted the Republicans’ economic plans with what he called the administration’s “balanced” proposal to include spending cuts with higher taxes on the wealthy by allowing the Bush-era cuts on the top income bracket expire.

“We’re gonna ask – yes – we’re gonna ask the wealthy to pay more,’’ he said. “My heart breaks.’’

“You know the phrase they always use?’’ he added, again lowering his voice, this time in imitation of a scary-sounding television attack ad. “Obama and Biden want to raise taxes by $1 trillion.’’

“Guess what,’’ he said. “Yes, we do, in one regard. We want to let that trillion-dollar tax cut expire so the middle class doesn’t have to bear the burden of all that money going to the superwealthy. That’s not a tax raise. That’s called fairness where I come from.’’

The Romney-Ryan campaign jumped on the remark, highlighting the “yes, we do” phrase to accuse the president of baldly embracing tax hikes. An image of a yes-we-do T-shirt was circulated by a campaign spokesman, and Mr. Ryan planned to highlight the remark at a joint rally with Mr. Romney in Virginia late Thursday.

But the administration’s proposal to increase revenues by some $1 trillion is fairly old news. The White House and Congressional Democrats have made no secret that is the estimated figure to be raised by allowing the Bush-era tax cuts for the highest earners to expire next year, which has been part of Mr. Obama’s agenda for two years.