An approximately $7.5 billion emergency spending bill to address the coronavirus outbreak in the United States is hung up in a dispute over vaccine affordability, lawmakers and aides in both parties said Tuesday.

Negotiations continued, with members on both sides of the aisle expressing optimism about finalizing the legislation quickly, hopefully in time to move it through the House and Senate by week’s end.

But hopes for releasing the bill Tuesday faded as negotiators squabbled over language Democrats want in the legislation that would ensure affordability of vaccines, such as by requiring reasonable prices and providing funding for the Health and Human Services Department to provide vaccines free, according to a House Democratic aide.

Some Republicans view the Democrats’ requests as price fixing and inflation caps outside the scope of precedent, which risks slowing development of drugs for vaccines and therapeutics, according to a House GOP aide.

Both aides spoke on the condition of anonymity to discuss the ongoing negotiations.

“We all believe we will have a bill passed and on the president’s desk by the end of this week,” Minority Leader Charles E. Schumer (D-N.Y.) told reporters, while adding: “We believe that the vaccine should be very cost-effective and not be out of the reach of average folks, and that’s one of the arguments that’s still being disputed.”

Majority Leader Mitch McConnell (R-Ky.) said he was optimistic that “we’re going to come together on a bicameral, bipartisan basis with a package. … I think we’re on the way to getting it through the House this week and with cooperation could get it through the Senate as well.”

President Trump also sounded bullish as he spoke during a visit to the National Institutes of Health.

“We‘re doing very well in terms of getting the funding we need, the necessary funding. I asked for X, and they want to give us more than X, and that’s okay,” Trump said. The initial funding proposal from the White House sought $1.25 billion in new money to fight the virus, plus $1.25 billion in repurposed funds from other accounts.