For the third time in six months, President Obama is turning to Vice President Biden to forge an agreement with Capitol Hill over government spending and taxes.

On Thursday, in the highest-stakes effort yet, Biden will convene a bipartisan group of lawmakers from the House and Senate to discuss raising the legal limit on how much the government can borrow and a longer-term plan to bring federal spending in line with revenues.

The meeting follows Biden’s work to find consensus within Congress earlier this year to avert a government shutdown and in a secret series of negotiations with Senate Minority Leader Mitch McConnell (R-Ky.) that sealed a December tax deal.

The White House says that one of Biden's key advantages is that while he speaks on behalf of the president, he also relies on his own stature and the personal relationships he developed over decades as a Delaware senator to test out ideas that may not be ready for formal negotiation.

“He has the ability to float trial balloons,” said Jack Lew, Obama’s budget director, in an interview. “He can say, ‘I’m going to go outside the official role and say, this is what Joe Biden thinks.’ ”

With the pressure on, Biden faces formidable challenges. Congressional leaders expressed skepticism when Obama announced last month that he wanted them to appoint 16 lawmakers to work with the vice president on an agreement. They named just six.

While they agree on the need for deficit reduction, there is a vast disagreement between Republicans and Democrats on how to accomplish that. And Biden’s effort is competing with the work of others in Congress trying to fashion a legislative compromise.

White House aides say Biden’s goal will not necessarily be to negotiate the weeds of a deal but to come up with broad parameters so that progress can be made quickly. Lawmakers in both parties say they want a specific plan for deficit reduction before agreeing to raise the debt limit. The Treasury Department warns that the United States could default on its debt by Aug. 2 if Congress doesn’t raise the limit.

“There is an urgency here,” said Bruce Reed, Biden’s chief of staff. “The debt ceiling cries out for a bipartisan, bicameral process because it’s a tough vote for everybody, and it’s not going to pass on the strength of one party or the other.”

How the talks will unfold isn’t clear, as both sides calculate where the other side will compromise.

Back in March, when congressional talks on a plan to fund the government for the remainder of 2011 had stalled, Biden visited Capitol Hill and summoned the leadership to his ceremonial office.

He banished staff to the cloak room for an hour as he talked alone with the leaders, saying that the Senate should vote on both the Republican and Democratic proposal. Biden believed that would allow the two sides to show their members they needed to compromise.

“He brings 36 years of legislative experience and can see how it looks through the other guy’s shoes,” a senior White House official said. “The key to a principled compromise is to figure out how both sides find common ground and hold their heads high.”

But the vice president’s efforts go only so far. Despite his work, the ultimate agreement required the intervention of Obama and went down to the wire. Some Republicans say they respect Biden but that the impact of his participation in the negotiations shouldn’t be overstated.

“I know they sell this story that Joe Biden is the hero of bipartisan negotiations and he’s so critical to getting this done,” said a GOP leadership aide, who pointed out that Biden left for a week-long trip to Russia just after visiting the Capitol. “There was very little face-to-face engagement with the vice president” on the 2011 budget negotiations, said the aide, who requested anonymity to avoid prejudicing the upcoming talks.

White House aides said Biden was engaged with the issue even while abroad on the long-planned trip.

Gaffe-prone and known for talking longer than desired, Biden has reined in those tendencies over the past two years. His background in the Senate was based in foreign relations — where he has also played a role in the White House — but advisers say he has stretched to master economic and budget issues.

“People are very focused on these relationships on Capitol Hill,” said Ron Klain, former chief of staff to Biden. “I think that what might be most surprising to people is how much he digs into the meat and substance of these issues.”

At the Blair House budget meeting Thursday morning, Biden will be joined by Treasury Secretary Timothy F. Geithner, Lew and National Economic Council director Gene Sperling. The meeting won’t be the last. Biden has largely cleared his calendar for meetings to hash out a deal over the coming weeks, aides said.