With his formal debut tonight in Denver as the Democratic nominee for vice president, Sen. Joseph Biden (Del.) faces a tall order from Congressional colleagues who expect him to introduce himself to the American people, hit hard against Republican presidential hopeful Sen. John McCain (Ariz.), make a plea for party unity and lay out key elements of a prospective Obama-Biden platform.

Biden, a nearly four-decade veteran of the Senate and expert on foreign affairs, will take to the Democratic National Convention stage at Denver’s Pepsi Center to deliver the final headline speech before Sen. Barack Obama (Ill.) accepts the Democratic nomination for president on Thursday. Before 4,200 Democratic delegates and thousands of onlookers, Biden will have the chance to prove his place as the No. 2 on the party’s ticket, by both attacking McCain’s credentials and shoring up Obama’s.

“He’s going to contrast the fact that John McCain voted consistently against middle-class families and with George Bush, not only generally but very specifically,” said Sen. Debbie Stabenow (D-Mich.), whose home state has been hit hard by the country’s economic crisis. “He’s going to create contrasts. He’s been serving with John McCain. He knows John McCain. He can very effectively talk about why John McCain is more of the same.”

Expectations for Biden are high, especially on a night that features a highly anticipated speech by former President Bill Clinton — the second major speech from a Clinton in as many nights, and one sure to feed media chatter about ongoing tension between the Clinton and Obama camps.

Biden will also be sharing the stage tonight with 2004 Democratic nominee Sen. John Kerry (Mass.), Senate Majority Leader Harry Reid (Nev.), and two other one-time possibilities for vice president, Sen. Evan Bayh (Ind.) and New Mexico Gov. Bill Richardson.

“He’s got a tough speech,” one senior Senate Democratic aide said. “Of the four speeches, [Clinton’s] has to be about our base and pulling together behind a great team, Biden’s has to be about why this team is better than John McCain’s vision for America. He has to show that he and Obama together are a powerful recipe for change in America.”

Obama’s selection of Biden as his vice president, which became official early Saturday morning, came after weeks of speculation over whether the junior Senator from Illinois would look to a seasoned establishment candidate or a new political face for his No. 2. As it turned out, Obama decided on the six-term Delaware Senator who unsuccessfully sought the 2008 nomination himself and, as chairman of the Foreign Relations Committee, is one of the Senate’s most notable national security voices.

Biden is expected to help balance Obama’s short political tenure with his lengthy one, while also helping to pad the presidential nominee’s résumé with his knowledge of military affairs, diplomacy and international relations. Despite his years on Capitol Hill, several Democrats said Biden remains relatively unknown outside the Beltway and will need to spend time tonight introducing himself to voters.

“A lot of people don’t know anything about Joe Biden,” said Rep. Artur Davis (D-Ala.), a Congressional adviser to the Obama campaign. “They know he ran for president, and they know he didn’t win. So I think Joe … will spend some time talking about who he is, his background, because biography is so important in modern American politics.”

Although viewed as a safe selection for Obama, Biden is not without his weaknesses, including a propensity for loquaciousness and the occasional gaffe. Still, Biden is respected on both sides of the aisle, and his colleagues in particular say he is among the better picks Obama could have made heading into November. Sen. Byron Dorgan (D-N.D.) predicted that Biden will “be very aggressive” and “be a very effective messenger” for the ticket, and that even beyond this week’s events, the Delaware Democrat will help Obama put into effect a plan to compete in previously GOP-dominated regions of the country.

“He’s already campaigned in all these states,” Dorgan said. Voters “already know him there. He brings a significant advantage to the campaign in that respect.”

Reid said in an interview Tuesday that he believes the addition of Biden — the fourth-most-senior Senator — will help “pick up all kinds of Democrats” for the campaign, including some long-standing Republicans. Asked what influence he had over Obama’s pick, Reid said: “I talked to Barack Obama several times about who he should not select — and that’s as far as I will take it.”

Democratic colleagues also said Biden can and must excel, tonight and beyond, in what has become a critical function for vice presidential nominees — that of attack dog. Some Democratic lawmakers and strategists have been wringing their hands this month as Obama’s once-solid lead in national polls has evaporated, blaming the slide in part on what they see as the campaign’s failure to strike back forcefully against attacks from the McCain camp. In Biden, they said, Obama has a heavyweight boxer with velvet gloves — someone who can stay on offense against the Republican ticket without flouting Obama’s promise to practice a new kind of politics.

“I think that Sen. Biden can draw out the starkest of policy contrasts between the Democratic ticket and Sen. McCain and yet do so, as he did on Saturday in Springfield, Ill., in the respectful tone of a colleague,” Rep. Tammy Baldwin (D-Wis.) said. Added Rep. Paul Hodes (D-N.H.): “One of his strengths is his ability to point out those differences with a sense of respect and good humor.”

Sen. Ben Nelson (D-Neb.) said Biden will have to strike just the right balance in tonight’s address, talking not only about party unity, but also stressing Obama’s themes of change and hope and outlining some specific proposals that would define a Democratic White House. Nelson said he also expects Biden to embrace his newfound role as the ticket’s chief attack dog, calling out McCain’s negatives and painting the “contrast between [a Democratic] administration of change and the continuation of the policies of the past.”

“Biden needs to talk about what they want to change — not only the tone of Washington and the direction of Washington, but also what they want to do with so many important issues facing America today, including our standing in the world,” Nelson said. “He’s the person to do it, he can talk about how Washington needs to change. He’s spent 30-plus years in Washington, and nobody knows more about Washington and John McCain.”

Emily Pierce contributed to this report.