WASHINGTON — As the Zika virus escalates into a public health crisis, members of Congress remain entrenched politically, with Republicans and Democrats pointing fingers over the failure to act as the number of mosquito-transmitted cases in the U.S. grows.

Health and Human Services Secretary Sylvia Burwell said Wednesday that her budget for fighting Zika is running out quickly. Without more money fast, she said, the “nation's ability to effectively respond to Zika will be impaired.”

Lawmakers left Washington in mid-July for a seven-week recess without approving any of the $1.9 billion President Barack Obama requested in February to develop a vaccine and control the mosquitoes that carry the virus.

Abortion politics played a key role in the gridlock over the anti-Zika bill. Republicans angered Democrats by adding a provision to a $1.1 billion take-it-or-leave-it measure that would have blocked Planned Parenthood clinics in Puerto Rico from receiving money.

Sen. Tim Kaine of Virginia, the Democratic candidate for vice president, has called for Congress to reconvene to immediately address the threat posed by Zika. Senate Majority Leader Mitch McConnell, in an op-ed published Monday in the Lexington Herald-Leader, criticized Democrats for balking at passing the bill.

Burwell's Aug. 3 letter seeks to counter Republicans who've criticized the Obama administration for not using several hundred million dollars already in the federal budget for Zika prevention. The money was initially allotted for fighting Ebola but was redirected to address Zika.

Burwell said her agency is committed to using “scarce federal dollars aggressively and prudently.” The Centers for Disease Control received the bulk of the $374 million “repurposed” for Zika domestic response efforts, she said, and it will exhaust the remainder of the money by Sept. 30.

Money for vaccine development will run out even sooner, she said. The second phase of clinical trials would be delayed as a result, and Americans would have to wait longer for a vaccine, according to Burwell.

“Now that the United States is in the height of mosquito season and with the progress in developing a Zika vaccine, the need for additional resources is critical,” Burwell wrote.

Zika is a looming economic development problem too, according to Sen. Marco Rubio, R-Fla. Many Florida businesses depend heavily on tourism and the state's economy could be hurt if potential visitors decide to stay away, he said.

Sen. Bill Nelson, a Florida Democrat, said Congress doesn't have to interrupt its lengthy summer break to pass the bill. Nelson's state has become the epicenter for Zika in the U.S. Fifteen people are reported to be infected with the virus in Miami's Wynwood arts district. These are believed to be the first mosquito-transmitted cases in the mainland United States, a situation that Nelson said heightens the urgency to respond.