In February, President Obama asked Congress for $1.9 billion in emergency funds to address the growing threat of the mosquito-borne Zika virus, which has been linked to serious birth defects, has already overtaken much of South America and the Caribbean, and is expected to threaten the United States this summer.

Two months later, Republican leaders in Congress have yet to act on the White House request, and Democrats are increasingly critical as summer approaches. On Thursday, a half-dozen Democratic senators gave floor speeches urging action, and party leaders called on Senate Majority Leader Mitch McConnell (R-Ky.) to cancel next week’s recess in order to tackle the issue.

“We need action now,” said Sen. Patty Murray (D-Wash.), the ranking Democrat on the Health, Education, Labor and Pensions Committee. “Women simply cannot afford to wait, and they should not have to.”

But Republicans are standing firm, arguing that the issue can be dealt with in next year’s spending process and, as Senate Majority Whip John Cornyn (R-Tex.) put it, would give a “blank check” to the White House to use funds meant for Zika for other priorities.

“It doesn’t take a lot of thought to realize that this is a request for a blank check without regard for the accountability that comes from . . . the appropriations process in the Senate,” Cornyn said. “What they want to do is play a shell game with this money. They want to get the money and if they don’t need it to deal with Zika, they can transfer it for other purposes, again without any transparency, without any real political accountability.”

Last week, there appeared to be a deal in the works among Senate appropriators in which a smaller sum, about $1 billion, would be provided in emergency Zika funding as an amendment to one of the 2017 spending bills now moving through the regular process. But that deal is now on ice, partly due to an unrelated blowup over the Iran nuclear deal and, it appears, internal GOP politics.

So why are Republicans so reluctant to pass new funding for Zika?

To understand the GOP opposition requires knowing a little bit about congressional appropriations and the internal politics of the Republican Party.

In the normal course, lawmakers deliberate for months, crafting detailed spending bills that meet previously agreed-upon budget caps and pass through both houses of Congress before the new fiscal year begins on Oct. 1. (Though, in recent decades, that deadline is rarely met.)

“Supplemental” funding, like that sought by the White House in the case of Zika, is different. It is meant to address emergency spending needs that occur in the middle of  a fiscal year. Those requests do not adhere to the same process as regular appropriations bills and, historically, they have not been subject to budget caps.

Republicans have thus far maintained that the Zika threat can be sufficiently addressed through the redirection of already-appropriated funds — at least until Oct. 1, when the next fiscal year’s appropriations kick in (at least, are supposed to kick in).

The White House has already moved to redirect $589 million in existing funds to Zika, most of it from Ebola response programs. Funds were also taken from other emergency preparedness programs, which has state and local jurisdictions warning that they may be unable to address other non-Zika threats.

The Obama administration and congressional Democrats continue to insist new emergency funding is necessary, but Republican appropriators — particularly in the House — are skeptical that additional dollars are necessary before the next appropriations season begins.

Rep. Hal Rogers (R-Ky.), the Appropriations Committee chairman, said last week that the White House is “refusing to provide basic budgetary information” on their Zika request. “This includes not answering our most basic question: What is needed, right now, over the next five months in fiscal year 2016, to fight this disease?”

Republicans have political reasons to resist the White House request, too. For one, taking up any spending bill — let alone an emergency bill outside of the regular process — has been a divisive exercise for GOP leaders ever since they won back the House majority in 2010.

Conservatives are already balking at any funding that isn’t offset with cuts elsewhere, and given the fact that it is an election year and many GOP incumbents are still facing primary challenges, a significant chunk of the Republican conference might oppose a spending bill of any kind. That would force GOP leaders to rely on Democratic votes, thus undercutting their pledges to respect the will of their own party’s members.

Moving an emergency spending bill creates another problems for Republicans: There’s no guarantee it would only include Zika funds. Democrats have pushed to include other priorities of theirs — including aid for the Flint, Mich., water crisis and help for opioid addicts.

Sen. Charles E. Schumer (D-N.Y.) said Thursday that Democrats are seeking to address all three issues, not just Zika: “We have more than one emergency in America, so I don’t get the logic of saying we should only do one.”

Murray on Thursday said the appropriators who had been willing to make a deal “have been beaten back by the extreme right wing who doesn’t want to do anything at all.”

“These extreme conservatives do not recognize that Zika is an emergency,” she said. “They don’t want to give our administration a penny more, and as a result of that delay, we are behind the eight-ball as mosquito season comes this summer.”