WASHINGTON – Lawmakers left the Capitol for the 4th of July recess Thursday without reaching resolution on a measure that would help combat the Zika virus.

When they return Tuesday, they’ll have less than two weeks to pass the measure, raising doubts of whether Congress will take action on the mosquito-transmitted virus before a seven-week recess stretching until after Labor Day.

Senate Democrats overwhelmingly rejected a House-Senate conference report that would have provided $1.1 billion for fighting Zika, including funds for vaccine development and diagnostic tests. They said the measure contained “poison pills” that took aim at Planned Parenthood and Obamacare funding.

Majority Leader Mitch McConnell said the Senate would revisit the legislation after the long 4th of July weekend. But Democrats say they won’t support the bill as it stands and want to reopen conference negotiations. Conference committees are supposed to reconcile Senate and House versions of legislation.

The partisan deadlock on the bill comes as public health officials warn that the U.S. could see an uptick in Zika infections due to the onslaught of mosquito season. Currently, 820 cases of Zika have been reported in the continental U.S., and nearly all but one was contracted during travel abroad, according to the Centers for Disease Control. That single person was infected in a laboratory. Zika has been linked to serious birth defects in children born to mothers infected with the virus.

Public health experts and researchers are increasingly frustrated by the political fight over Zika funding. Anthony Fauci, the director of the National Institute of Allergy and Infectious Diseases at the National Institutes of Health said that without new money, efforts to develop a Zika vaccine would be stopped in their tracks.

“The vaccine effort will be blunted, if not completely aborted, if we don’t have the money to go into the advanced trials,” Fauci said told reporters Wednesday.

What’s more, failing to approve a plan for the fight against Zika could prove politically dangerous. Norm Ornstein, resident scholar at the center-right American Enterprise Institute called the deadlock over Zika legislation “a game of political chicken.”

“At this point there’s a 50-50 chance of anything getting done,” Ornstein said in a phone interview. “There’s a huge impetus to act, but this isn’t a Congress that can shoot straight.”

The House passed the Zika conference report 239-171 on June 23 amid a sit-in by Democrats protesting a lack of action on gun control legislation. The symbolic move in the House chamber prompted Republican leaders to break early for the holiday recess but not before a rushed vote on the Zika bill. The measure won the support of only six Democrats.

No Democrats signed the bicameral conference report which must be approved by both chambers of Congress before the bill can be sent to the president. Senate Minority Leader Harry Reid said on the Senate floor Tuesday that Democrats – a minority in both the Senate and House — were left out of negotiations on the final version of the legislation.

Both parties are blaming one another for politicizing a public health crisis. Senate Democrats fault Republicans for using the legislation to promote a conservative agenda and question why funding for Zika was included in the Veterans’ Affairs spending bill – the larger measure before the Senate. Republicans are accusing Democrats of pulling back on legislation they voted for last month.

After a Senate Homeland Security and Governmental Affairs committee meeting on Zika Wednesday, Sen. Ron Johnson, R-Wis., lambasted Democrats for “playing politics with a very serious issue.”

“I don’t quite get it,” Johnson said, adding, “Hopefully they’ll go on break and they’ll reconsider and they’ll come back and support what the House has already passed.”