By David Morgan

WASHINGTON (Reuters) - The White House and congressional Democrats pressured Republicans on Thursday for bipartisan Zika-funding legislation, saying the public health battle against the mosquito-borne virus is being undercut by efforts to ram through a bill with less funding.

But there was no sign that Republicans would abandon their $1.1 billion measure, raising the chance that Congress will leave the growing health crisis unattended until September.

In a conference call, the director of the Centers for Disease Control and Prevention said the lack of funding has made it difficult for the health agency to ramp up mosquito-control and diagnostic testing and pursue new research.

President Barack Obama asked the Republican-controlled Congress in February for $1.9 billion in emergency funding to fight Zika. But as lawmakers prepare to leave for a seven-week recess on July 15, Senate Democrats who defeated the Republican measure last week predict it will only fail again.

"We are at the 11th hour and 59th minute before Congress is gone all summer," said Democratic Senator Bill Nelson of Florida, which is among a number of southern U.S. states likely to be hard-hit by the Zika virus.

U.S. health officials say Zika infections in pregnant women can cause microcephaly, a birth defect marked by small head size that can lead to severe developmental problems in babies.

Over the past few days, Obama has urged Senate Republican leader Mitch McConnell and Senate Democrats to adopt a bipartisan approach. On Thursday, Democrats called on McConnell and House of Representatives Speaker Paul Ryan to revive a bipartisan measure that passed the Senate in May.

McConnell has said he would stick with the current legislation, which was agreed by House and Senate negotiators and has already passed the House.

A senior Senate Republican aide dismissed the call for change as "cover" for Democratic efforts to block the Republican measure. A Ryan spokeswoman said: "We hope Senate Democrats do the right thing and end their filibuster."

Even some Democratic aides expressed skepticism that McConnell or Ryan would change course.

The CDC is monitoring 320 pregnant U.S. women with laboratory evidence of Zika infection. In Puerto Rico, where Zika has been rampant for months, as many as 50 pregnant women are infected every day, CDC Director Thomas Frieden said.

Altogether, there are more than 1,130 Zika cases in the United States, the CDC said. All contracted the virus outside the United States, in a lab or through unprotected sex.

Frieden said the CDC has spent most of the $222 million it has received for Zika in the United States on state and municipal efforts, funding, staffing and equipment.

"It's frankly difficult to navigate with so many unknowns," Frieden said. "That's why we haven't been able to do things like begin ramping up centers of excellence to improve the performance of diagnostic tests, of mosquito control and to begin some of the really in-depth studies."

Additional funding would also aid vaccine research and help community health centers in areas with the highest Zika transmission rates.

The Republican measure failed in the Senate on June 28 when Democrats objected to Republican provisions, including one to prevent Planned Parenthood from receiving funds to combat what can be sexually transmitted infection.

The legislation would also take money from battling the Ebola virus and from funds set aside to implement the Obamacare health insurance program in U.S. territories.

"They'll force yet another failed vote on this cynical legislation and then pack their bags for the longest Senate vacation since 1954," Senate Democratic leader Harry Reid said.

The World Health Organization has said there is strong scientific consensus that Zika can also cause Guillain-Barre, a rare neurological syndrome that causes temporary paralysis in adults. The connection between Zika and microcephaly first came to light last fall in Brazil, which has now confirmed more than 1,600 cases of microcephaly that it considers to be related to Zika infections in the mothers.

(Reporting by David Morgan; Editing by James Dalgleish, Bernard Orr)