President Obama's administration has almost $21 billion available to fight the Zika outbreak, despite its claims that it is running out of money for the crisis, according to a pair of senators.

Zika funding has been a political football throughout the summer, as Senate Democrats filibustered a bill redirecting $1.1 billion to fight Zika funding because the legislation didn't fund Puerto Rico's Planned Parenthood affiliate, and because Democrats wanted to fund the effort with deficit spending.

In the ensuing weeks, the two parties have sought to shift blame for the lack of funding and pressure the other side to relent. Health and Human Services Secretary Sylvia Burwell wrote a letter to Congress stating that the department was running out of money to fight Zika, but Republicans say that's not correct.

But HHS is projected to have $20.996 billion in "unobligated balances" from a fund originally created to finance "bilateral economic assistance to foreign countries" that could be redirected to the Zika fight, two top Republicans reminded Burwell in a Friday letter that was released publicly on Monday.

"Has your administration used the authority provided to you in [federal law] to transfer moneys under the bilateral economic assistance heading toward a response to the Zika virus?" Sen. James Lankford, R-Okla., and Sen. Chuck Grassley, R-Iowa, asked in their letter.

"If not, please explain why and whether you have plans to do so, and if you believe every dollar spent in bilateral economic assistance to foreign countries is more important than protecting Americans from the Zika virus."

That emphasis on foreign aid funding could give Republicans an effective counterattack against Democrats who have said Congress' failure to fully fund the Zika fight aren't fit to govern.

The fight has also given one crucial incumbent GOP senator the opportunity to rehabilitate his image following a bruising defeat in the Republican presidential primaries.

Florida Sen. Marco Rubio has taken an aggressively bipartisan approach to the Zika debate, which gives him a chance to serve constituents in a health crisis after months of being accused of ignoring his state during the presidential campaign.

"The time for this political gamesmanship has to end," Rubio said in August. "It is inexcusable that the president of the United States and this administration is sitting on over $300 million that were repurposed from Ebola to Zika that remain unspent.

"By their own admission, at least $175 million of federal funds are immediately available to be spent, and I do not understand why that money isn't being spent right now," he added. "Now, I don't like to ascribe motives, but in this case I have to start wondering whether they are holding back on spending that federal money because they like making this a political issue that they can attack Republicans on."