Senate leaders had hoped to complete action on the legislation next week and send it to the House but now that ambitious timetable might be delayed if they can't reach a deal soon.
"It's a work in progress," Senate Minority Leader Harry Reid, D-Nevada, told reporters. "A lot of work to be done on that."
Sen. John Cornyn of Texas, the second-ranking Senate Republican, sounded a little more optimistic an agreement would be struck soon but said the closed-door negotiations -- especially regarding Planned Parenthood funding in the Zika bill -- are at a sensitive point.
'We're that close," Cornyn said holding up two fingers an inch apart. But, he added, "nothing is decided until everything is decided so that's why I'm hesitant to talk publicly about it but it's getting closer."
With just under eight weeks until Election Day and with control of the Senate in doubt, Senate Republican leaders are anxious to complete their work and send their vulnerable members up for re-election home for a final campaign push. Democrats are keenly aware of that desire and recognize it might provide them some leverage in the late stages of the negotiations.
"I think they'd love to have us here longer than shorter," said Sen. John Thune, R-South Dakota suggesting Democrats might want to slow-walk talks to keep senators in DC. "It's advantageous to them to have our incumbents here."
Another key deadline is September 30, when a deal on federal spending must pass or agency operations could be shut down. GOP leaders very much want to avoid that scenario this close to the election because they worry voters will blame Republicans -- who have majorities in both chambers -- for not governing responsibly.
A key sticking point is language in the Zika bill that keeps funds in the bill from going to clinics in Puerto Rico that are associated with Planned Parenthood. Democrats are insistent that restriction be removed and Republican leaders have acknowledged they must change the language to get enough Democratic support to pass the bill. But to this point, negotiators haven't been able to agree on language that would be acceptable to both sides.
"Planned Parenthood is not gone yet," Reid said tersely.
Florida Gov. Rick Scott, whose state is at the center of the Zika epidemic, traveled to Washington and told reporters he's met with House Speaker Paul Ryan, other top leaders, Senator Marco Rubio and many members of the Florida congressional delegation to press for more money for prevention kits, vaccine research and mosquito abatement.
"The truth this is not just a Florida issue, this is a national and international issue," said Scott, a Republican. "This is something everybody cares about -- developing babies. We want to make sure we take care of them."
He sounded upbeat that a deal would emerge, adding, "I'm up here because I want Congress to act soon. I want them to act today. I know there's a whole process up here but I know everyone I've talked to has been very, very supportive."
Internet domains, Export-Import Bank, Syria among issues involved
Other controversial issues have creeped into the negotiations: Sen. Ted Cruz wants to add language to stop the Obama administration from releasing control of Internet domain names to an international consortium that includes Russia and China, fearing those countries will stifle Internet freedom.
Business groups are pressing Congress to make functional changes to the Export-Import Bank, a difficult task because many conservatives decry the bank as an example of crony capitalism.
Environmental groups are concerned about damage caused by the pesticides that are used to kill Zika-carrying mosquitoes.
House conservatives want to add language barring refugees from Syria and other countries with terrorist ties from entering the US -- a provision that would be a non-starter for Democrats.
And states that have been damaged recently by heavy flooding are pressing for federal aid to be added to funding resolution.
House Appropriations Chairman Hal Rogers called the negotiations over the funding and Zika package "promising."
Rogers told reporters he received a letter from the White House notifying him that the Obama administration was putting together a new request for $2.6 billion in funding for flood recovery efforts in Louisiana.
Asked if the money for Louisiana could be added to the funding bill, Rogers said, "It's going to be really tough to do that because we don't know the details yet," but he didn't rule it out.
In the House, both Democratic and Republican aides told CNN that the negotiations are proceeding on several tracks, and there is progress, but it's unclear when a final bill would be unveiled. They anticipate the Senate leaders will move first, and give Ryan little time or choice but to move the package to the House floor, where he will need to turn to Democrats to help pass it.