Washington ? Under an urgent deadline, President Barack Obama and congressional negotiators set their sights on the nation’s tax system and cherished benefit programs Thursday in hopes of striking a budget deal to raise the nation’s debt ceiling. Obama said the two sides were still far apart and called everyone back to the White House on Sunday.

The president met with the eight top Republican and Democratic congressional leaders for an hour and a half Thursday, hoping to bridge ideas held by the two sides — each considered untenable by the other. On Capitol Hill, Democrats appeared especially rattled that the discussions included proposals to cut spending for Social Security as well as Medicare and Medicaid.

High-level talks, after dragging on for weeks, have entered a suspenseful endgame. The shape of an agreement is still in doubt as the nation moves ever closer to an Aug. 2 deadline to raise the government’s debt ceiling.

Obama said Democrats and Republicans should be prepared to show their bottom-line demands when they return to the bargaining table for the rare Sunday session.

The negotiating stakes are high. Without a deal on deficit reduction, Republican leaders say they don’t have enough GOP votes to increase the nation’s borrowing authority, raising the danger of the first ever U.S. default on its debts when the current $14.3 trillion debt ceiling is tapped out.

“Everybody acknowledged that we have to get this done before the hard deadline of Aug. 2 to make sure that America does not default for the first time on its obligations,” Obama said. “And everybody acknowledged that there’s going to be pain involved politically on all sides.”

That leaves little time to agree on 10-year deficit reductions of $2 trillion to $4 trillion.

The major clash centers on how to reduce spending on major entitlement programs such as Medicare, Medicaid and Social Security, all prized by Democrats, and on tax changes that would close loopholes and end certain corporate breaks. Republicans insist that any tax changes be used to lower rates on corporations and individuals; Obama wants them also to generate more tax revenue.

Increasing the debt ceiling through the end of 2012 — a date favored by the White House — would require authorizing about $2.4 trillion in additional borrowing. House Speaker John Boehner has insisted on a 10-year deficit reduction figure that, at a minimum, matches the amount of additional borrowing. One aide to a lawmaker in Thursday’s meeting said Obama made it clear he wouldn’t sign a budget and debt agreement that didn’t extend the debt ceiling until after the November 2012 presidential election.

In the meeting, Obama told the leaders that they faced three options — a small deficit reduction plan, a medium plan that would reduce deficits by $2 trillion over 10 years or a big agreement that would shoot for up to $4 trillion in deficit reductions over the next decade. Obama indicated he preferred the largest number.