Biden, the sometimes off-script vice president, is set to address the thousands of delegates here — and millions more watching at home — during his appearance Thursday evening. Advisers say Biden will tell what he saw from his front-row seat as Obama made some of his hardest decisions. And he’ll make the case that Republican rival Mitt Romney isn’t suited to make the tough calls.

His target audience will be the white, working-class voters who will be seeing a lot more of the vice president in the coming weeks.

“That’s his base,” said Ted Kaufman, a longtime Biden confidant who took his seat in the Senate when Biden was elected vice president in 2008. “He’s letting people know he understands their problems.”

Look for Biden to use words such as “conviction,” ”character” and “values” in a speech honed for middle-class voters, his advisers said. He also plans to talk about the decisions that have defined Obama’s first term in office, such as sending Navy SEALs into Pakistan for the fatal raid on al-Qaida leader Osama bin Laden and helping bail out Detroit’s automakers.

“Osama bin Laden is dead and General Motors is alive,” Biden says regularly in his campaign appearances.

Biden also plans to talk about the respect he has developed for Obama during the past 3½ years, particularly the president’s hands-on approach to foreign policy. The two sometimes have disagreed, but that has only increased Biden’s standing with Obama, who appreciates discussion over dictating decisions. On days they are both on White House grounds, they spend some four hours together in meetings; Biden often is the last person Obama consults on major decisions.

“He plans to speak about the president in a way that only he can,” said a person close to the vice president. The person requested anonymity because the individual was not authorized to discuss preparations ahead of the speech.

Biden also has been something of a headache for Obama. On the day Obama signed the Democrats’ health care overhaul into law, Biden stole headlines by using an expletive in range of a live microphone. He forced Obama’s hand on gay rights during an interview that sped up the president’s endorsement of gay marriage. And more recently, to an African-American audience in Virginia, he said of Republicans, “They’re going to put y’all back in chains.”

Republicans have worked to cast Biden as a gaffe-prone figure who has hung around the political scene long past his time. They have tried to paint the 69-year-old as a political liability.

The criticism has stuck. When the Pew Research Center/Washington Post poll asked voters for their one-word impression of Biden, more people use negative than positive words to describe him. Many of the terms disparage Biden’s competence and performance, with “idiot,” ”incompetent” and “clown” among the terms used most frequently.

Polling suggests Biden fares about as well as Obama among white, working-class voters — which isn’t saying he has a particularly broad appeal. Romney has the upper hand among those voters.

Yet Biden has a knack for connecting with blue-collar workers that Obama simply does not. He can deliver scathing criticism through clenched grins in a way that Obama cannot. He can promote Obama’s accomplishments that would sound like bragging if the president talked in the same way.

Plus, he has demonstrated a certain glee in eviscerating his rivals’ proposals.

“There is nothing gutsy about giving another trillion dollars in tax cuts to millionaires. There is nothing bold about turning Medicare into a voucher system,” Biden says in his typical speech.

Born in Scranton, Pa., and raised as a member of the working class, Biden speaks with credibility to voters’ frustrations with Washington, despite having won election to the Senate in 1972. He can move an audience with stories about seeing his father forced to move to Delaware to find work or coping with the death of his first wife and daughter in a car accident.

Biden’s speech was slated for 9 p.m. Thursday, before most television networks were scheduled to broadcast live coverage and before Obama speaks. Biden advisers note that the last running mates to tag-team speeches on the same night were Bill Clinton and Al Gore in 1992. They say the decision to put Obama and Biden together in prime time underscores the partnership they have developed.

It’s that package deal that Obama’s strategists hope voters will buy.