By Yuka Hayashi and Natalie Andrews

WASHINGTON -- The Senate was unable to finalize a deal to extend the amount of time companies have to spend loans obtained through the Paycheck Protection Program, putting off the likely passage of revised small-business aid rules to next month.

Amid broad bipartisan support, senators worked on Thursday to coalesce around a plan to double the time period to 16 weeks, but failed to garner unanimous consent on the agreement before leaving for a Memorial Day recess. The PPP is intended to help small businesses keep workers employed and pay other expenses during the coronavirus pandemic.

Under the current rule, the earliest recipients of PPP funds must finish using them by May 29. Senators also sought to extend the deadline for program applications to Dec. 31 from June 30, and allow businesses to use funds to pay for investments needed to reopen safely and buy personal protective equipment for employees.

"I don't think we're going to have a problem getting something done one way or the other on it," said Sen. Marco Rubio (R., Fla.), the chairman of the small-business committee.

Some senators had hoped to pass new PPP legislation before the break, even as GOP leaders have urged a go-slow approach on a broader aid package.

On Wednesday, Sen. Cory Gardner (R., Colo.) said it was "unfathomable" for the Senate to leave for recess without passing coronavirus-related legislation, adding that he would block the Senate from adjourning. Mr. Gardner, who faces a tough re-election fight, said Thursday that the Senate was "very close to a number of things that are needed" and didn't object to the Senate adjourning.

Separately, House Democrats are expected next week to vote on a bill to change the $660 billion program's time frame, and change some of the repayment terms. The House proposal has support from the U.S. Chamber of Commerce, the National Restaurant Association, the National Retail Federation and several other outside groups.

To become law, either bill would have to pass both chambers and be signed by the president.

The program requires businesses to put 75% of its funding toward keeping workers on the payroll for the loan to be forgiven, and to do so within eight weeks. But that has proven difficult for many businesses -- such as restaurants and hair salons -- that have remained closed and have little or no work to offer employees.

Demand for the program has cooled considerably in recent weeks, with new funds approved at a rate of roughly $1 billion a day. According to Small Business Administration data, a total of $513 billion had been approved under the program by Tuesday, leaving about $130 billion still available after accounting for lender fees. The government pays fees to banks for issuing the PPP loans, which are guaranteed 100% by the government.

An SBA spokesman said funds returned by some lenders have been added back to the pool, bolstering availability. He didn't disclose the amount of returned funds, but there is a gap of roughly $24 billion between the agency's earlier loan-approval data and the latest accounting. The spokesman said the latest number reflects cancellations of duplicative loans, loans that didn't close for whatever reason and loans that have been paid off.

The House plans to vote next week on a bill sponsored by Rep. Dean Phillips (D., Minn.) and Rep. Chip Roy (R., Texas) that would adjust the PPP, including extending the loan period and eliminating the requirement to use 75% of funds on payroll expenses.

"It has a sense of urgency about it," House Speaker Nancy Pelosi (D., Calif.) said of Mr. Phillips's proposal in talking to reporters on Wednesday. "What it does is extend the time in which you can rehire people, extend the time in which you pay back and also undo the 75/25, which was debilitating," referencing the requirement that 75% of loan proceeds be spent on payroll costs.

In comments Thursday at an online event hosted by the Hill publication, Treasury Secretary Steven Mnuchin said he supports extending the eight-week period and defended the administration's handling of the PPP.

He said the Trump administration rule requiring recipients to use 75% of the money on payroll costs, rather than rent or bills, was consistent with the law passed by Congress. Companies struggling to meet additional overhead costs, he said, can apply for other loans through the SBA.

Mr. Mnuchin also expressed reluctance to disclose PPP loans that are made to privately held companies, citing "proprietary information" concerns.

Write to Yuka Hayashi at yuka.hayashi@wsj.com and Natalie Andrews at Natalie.Andrews@wsj.com