Republican are calling on Democrats to pass a Zika funding measure as soon as tomorrow, when the Senate is scheduled to hold a pro-forma session.

"If Senate Democrats want to pass a bill now … they can simply give unanimous consent to pass the conference report in tomorrow's session and send it straight to the president," Don Stewart, spokesman for Majority Leader Mitch McConnell, R-Ky., told the Washington Examiner.

Earlier Thursday, Senate Democrats sent a letter to McConnell and House Speaker Paul Ryan, R-Wis., calling on them to reconvene Congress in order to take up a Zika funding measures. The demand follows news that mosquito-transmitted Zika virus has been discovered in a small section of Miami.

Neither chamber is otherwise expected to be in session until Sept. 6.

Zika normally causes a mild illness but can cause a birth defect called microcephaly in which babies are born with abnormally small heads and underdeveloped brains.

Republicans make the case that Senate Democrats are to blame for the funding holdup because they blocked $1.1 billion measure that had already passed the House and needed only the approval of the upper chamber to reach President Obama's desk.

Senate Democrats voted to filibuster the measure, mainly because it derived the funding from other government accounts. For example, it siphoned $543 million from a defunct Obamacare account that was to be spent on establishing health insurance exchange programs in U.S. territories. Another $100 million was to be shifted from a fund established to battle the Ebola virus, which is no longer a threat in the United States.

Democrats argued the GOP-authored bill included "poison pill" provisions, such as one that would have prevented new health care funds aimed at stopping Zika from being used at Puerto Rican Planned Parenthood clinics.

Senate Democrats, in the letter, demanded the GOP-led chambers to instead pass a Senate-authored bill that excludes the so-called poison pill provisions and does not cut from elsewhere in the budget.

"The simplest course of action would be to pass the Senate's clean bipartisan compromise on Zika funding by unanimous consent and have the House pass the same bill immediately," Democratic senators said in the letter to Ryan and McConnell.

Stewart suggested the GOP has no plans to re-open the Zika debate and noted that the Senate has several pro-forma sessions scheduled if Democrats aren't ready to act on the GOP bill tomorrow.

"Whenever they're ready to stop blocking funding for anti-Zika efforts and funding for our Veterans, we'll be here waiting," Stewart said.