Still divided after weeks of effort, Congress returns today to negotiations aimed at breaking a deadlock over balanced-budget legislation. Failure could lead not only to a government shutdown but to an unprecedented financial default by the federal treasury at week's end.

Fiscal deadlines, with predictions of dire consequences, are nothing new for Congress, which has sometimes been unable to act without time pressure. But rarely have legislators faced so many high-stakes deadlines over such a short time, with so little in the way of clues to the eventual outcome.

Adding to the deadline pressure, a financial default and resulting government chaos would occur at the worst possible time for President Reagan -- on the eve of his departure Saturday for his summit with the Soviets.

Rep. James R. Jones (D-Okla.) yesterday suggested a solution -- a temporary reprieve from the financing crisis to be followed by a "domestic economic summit" after Reagan returns from the summit.

But White House officials, at least, were publicly cold to the idea. A spokesman said the administration is still demanding action by Thursday. "It is up to them to act," said Edward P. Djerejian, calling for accord by then.

For more than a month, Congress has argued over legislation to extend the federal debt ceiling, an action legally necessary to give the government the authority to continue borrowing to finance its operations. It has attached the debt bill to new, controversial legislation that would force a balanced budget over the next four or five years by requiring the president to cut spending that does not meet fixed deficit targets.

Despite intensive negotiations, the political scorecard on the Gramm-Rudman-Hollings legislation is, if anything, more confused. The Republican Senate has approved one version, and the Democratic House has approved another that protects more programs for the poor, could cut defense more and requires deeper cuts in 1986, an election year for 22 Republican senators.

Going into the new round of talks, the White House is condemning the Democratic version and has private doubts about the Republican version. Over the weekend, Reagan said he did not want to go to the Geneva summit "with any new defense cuts in my hip pocket."

Distracted by this and other problems, Congress has also failed thus far to pass most of its regular appropriations bills that allow government agencies to spend money during the fiscal year that began Oct. 1. To keep the agencies in spending money, one short-term "continuing resolution" has already been approved; another must be approved by Thursday. That is necessary even if the debt-ceiling problem is solved or postponed.

Other programs, ranging from the cigarette tax of 16 cents a pack to dairy price supports and aid for firms and workers harmed by imports, expired Sept. 30 without any agreement on provisions for extension. They have survived with interim authority from Congress.

Both the continuing resolution for government agencies and the temporary program extensions expire on Thursday. And on Friday, the government must make a $16 billion payment of interest on the national debt, far exceeding the resources that the Treasury Department was able to tap as it searched for usable cash over the last few weeks.

Failure to meet the Thursday continuing resolution deadlines would leave the government without cash to operate, a situation that has occurred for brief periods. But failure to meet the debt-ceiling deadline would leave the government without power to borrow and facing financial default for the first time, according to the Treasury Department.

The stopgap spending bill also got trapped in deficit politics but the House is scheduled to vote on some aspects of the proposals today, and the administration is still predicting passage by Thursday.

However, the debt situation is a different story. White House reservations about potential defense cutbacks have added complications and politics to a situation awash in those elements. Some Democrats yesterday were predicting that Congress could reach an agreement, but not one that the president would likely accept.

Sen. Sam Nunn (D-Ga.) said, "The White House is looking for a reason to get out of this. If given half a chance, they'll pull the rug out from the whole thing."

Sen. Lawton Chiles (D-Fla.) added, "If the administration does not get in it, I think something will emerge . . . . There was a lot of suspicion that each side had toward the other last week, but if we get beyond the suspicions, I think that the House is realistic enough to know that they are going to have to strike a deal at some point."

The choices are difficult given the political complexities of the alternatives. Among them:

The House and Senate could reach a compromise on the balanced-budget legislation. But agreement eluded the two chambers in a previous round of negotiations and the dispute seems to have deepened.

Congress could forget about deficit politics and pass a straightforward extension of the debt ceiling. But deficit politics has become so compelling that none of the principal players wants to be the first to blink.