Seeking to draw a direct contrast between Romney and Obama, Biden said: "The president didn't flinch. This is a man with steel in his spine."
Thursday's speech marked the first of four general election events Biden will hold in the coming weeks as the Obama campaign takes sharper aim at the Republican field.
"Mitt Romney, Rick Santorum and Newt Gingrich have a fundamentally different vision," Biden said. "We're about promoting the private sector. They're about protecting the privileged sector."
Biden was greeted at the union hall with chants of "Go Joe, Go!" and "Four More Years." The crowd behind him held Obama-Biden 2012 signs.
"I'm back. You're back," Biden said as he revved up the crowd.
With Biden now fully engaged in the campaign, the Obama re-election team sees an opportunity to rely on a strong and forceful defender of the president to build support among Democrats, while allowing Obama himself to stay above the political fray for as long as possible.
While the three campaign principals -- Obama, Biden and first lady Michelle Obama -- have been headlining campaign fundraisers for several months, officials say the president won't hold campaign-sanctioned public events until after Republicans pick their nominee.
But many of the events Obama has held in recent months, which the White House has said are "official," have sounded like campaign events, including nameless references to the GOP candidates vying to replace him. That goes for a speech Obama delivered Thursday in Maryland, fighting back against Republican criticism of his energy record.
The first lady's campaign strategy is still being crafted, though she is expected to play an active role in the general election.
Biden's campaign strategy has been in the works for several months, and its early rollout underscores his importance to the Democratic ticket.
Heading in to the November election, Biden is expected to target the big three political battlegrounds -- Ohio, Pennsylvania and Florida. Obama carried all three in 2008, but will face an uphill climb in each come November given the toll the recession has taken on those states.
The campaign's goal is to use the vice president's strengths to counteract Obama's perceived weaknesses.
The president sometimes struggles to connect with Ohio's and Pennsylvania's white working-class voters. Some Jewish voters, who make up a core constituency for Florida Democrats, view him with skepticism. Biden has built deep ties to both groups during his four decades in national politics, connections that could make a difference.
As a long-serving member of the Senate Foreign Relations Committee, Biden cemented his reputation as an unyielding supporter of Israel, winning the respect of many in the Jewish community. And his upbringing in a working-class Catholic family from Scranton, Pa., gives the vice president a valuable political intangible: He empathizes with the struggles of blue-collar Americans.
Democratic officials also believe Biden's presence in key battlegrounds can benefit congressional candidates in those states as the party looks to regain some of the seats it lost in the 2010 midterm elections.