The Senate yesterday defied warnings that it was inviting a constitutional crisis surpassing Watergate as it marked time for a third day on urgent legislation that is needed by midnight Monday to extend the government's borrowing authority.

Voting 53 to 25, it rejected a proposal calling for a constitutional amendment to give the president stronger veto powers over spending--the so-called "line-item veto" of spending bills that President Reagan has frequently urged.

But Majority Leader Howard H. Baker Jr. (R-Tenn.) sternly warned the Senate that it would be granting President Reagan the "ultimate executive power" over the government's purse if it let the debt ceiling deadline go by and left it for Reagan to decide which checks go out and which checks bounce.

The warning had little if any effect as the Senate, called in for a rare Saturday session on the debt bill, spent most of its time on other matters, ranging from restrictions on press coverage of Grenada to funding of a Jordanian rapid deployment force.

"We have a major problem here," said an increasingly impatient Baker as it appeared that the Senate might not make the Monday deadline, either through delay or through outright opposition to the debt ceiling extension.

At another point, Baker acknowledged the possibility--some senators say probability--that the Senate will deliberately plunge the government into a credit crisis in order to force consideration by both Congress and the administration of major deficit-reduction measures.

"I'm beginning to get the feeling that the Senate doesn't want to pass the debt limit," said Baker, warning his colleagues that the result could be a constitutional crisis that would make Watergate look pale by comparison.

"Who decides which checks go out?" asked Baker, warning his turf-conscious colleagues that, if the answer is the president, Congress would be giving Reagan the "ultimate impoundment power" to spend only what he chooses.

This was only shortly after Sen. Alan J. Dixon (D-Ill.), backed by several conservative Republicans, tried unsuccessfully to get the Senate to go on record, in connection with the debt bill, as favoring a constitutional amendment to allow the president to veto individual items in spending bills. At present a president can only veto a bill as a whole.

Dixon noted that governors of 43 states already have such line-item veto power. He said there should be someone in Washington "with an eraser" to wipe out spending excesses that are approved on Capitol Hill.

Responding to objections that this would mean a loss of turf by Congress, Sen. Mack Mattingly (R-Ga.) said, "That's ridiculous . . . it's the taxpayers who are losing turf."

Minority Leader Robert C. Byrd (D-W.Va.) responded that Congress has already yielded enough powers to the president and added: "We ought to be very careful and make haste slowly in granting to any president powers he never had under the Constitution."

While the idea was "ingenious and innovative," said Baker, "I think it would make a fundamental change in the way the Congress works with the president" and should be addressed only after more careful study.

On the debt ceiling bill, Baker conceded that the government probably has enough cash to last a few days, but not much longer.

But Finance Committee Chairman Robert J. Dole (R-Kan.) cited a letter from Treasury Secretary Donald T. Regan that said the government could lose $250 million in higher interest costs if it hits its debt limit on Tuesday without new borrowing authority.

The House has already agreed to raise the current debt limit of $1.389 trillion to $1.615 trillion, enough to last through most of the current fiscal year ending Oct. 1, 1984. But the Senate has whittled the total down to $1.450 trillion, enough to last only through mid-February, and may not even pass that by the midnight Monday deadline.

Some senators said yesterday that that would be fine by them, provided it forced the president and Congress to consider deficit reductions to trim the national debt. But there seemed to be no consensus on the mix and extent of spending cuts and tax increases that would be necessary to do the job.

Even if the Senate leadership had the votes to pass the bill, and there were doubts about that yesterday, there was serious question whether the bill could be wrapped up and sent to Reagan for signature by Tuesday.

Dole said it would be impossible to finish a House-Senate conference on the measure by Monday night. He cited other work that the Finance Committee must complete, including relatively modest deficit reductions mandated by the budget that Congress adopted earlier in the year.

Dole suggested a 15-day extension to give Congress time to finish work on a long-term debt limit extension.

Baker was reluctant to do that, although he said he might have to in the end. Sen. William L. Armstrong (R-Colo.), an advocate of using the debt measure to force deficit reductions, warned that a bob-tailed debt extension might face the same opposition as the regular bill.

Congress usually waits until the last minute to approve debt limit increases and often lards them with other legislative proposals in what Dole sardonically called a "turkey shoot" during yesterday's debate.

But this year is different because of mounting congressional frustration over inability to deal with annual deficits approaching $200 billion a year and because of new laws governing investment of Social Security funds. Under a new statute, the Treasury Department is required to turn $13 billion over to the Social Security system on Tuesday. In addition, auction of government securities scheduled to start on Tuesday would have to be postponed, and this could lead to higher interest rates.