House Speaker Kevin McCarthy is pictured at the Ronald Reagan Presidential Library, in Simi Valley, California, on April 5.
CNN  — 

House Republican leaders are still struggling to get to 218 votes on their package to raise the national debt limit – and are uncertain when they will be able to hold the vote, according to multiple GOP sources.

While Republicans hadn’t yet scheduled the vote, they had made Wednesday their informal goal to lock down the votes and push their package through along party lines. It’s now unclear when the vote will occur.

GOP leaders are warning their members that if they sink the bill, it would give President Joe Biden, who has thus far refused to engage in negotiations over raising the nation’s borrowing limit, the upper hand in the high-stakes fight. Biden issued a veto threat for the legislation on Tuesday.

Republican leaders remain confident they’ll get the votes. But House Speaker Kevin McCarthy can only afford to lose four votes in the narrowly-divided chamber, and GOP leaders are facing resistance on several fronts within their own ranks.

“We will let you know,” McCarthy said Tuesday when asked if he would be forced to change the bill to win over skeptical Republicans.

McCarthy later opened the door to pushing back the timeline for when the House may vote, saying that the vote will come “this week” instead of locking the vote in for Wednesday.

“Vote on it this week. Yes,” he said.

On Tuesday evening, several House Republicans voiced opposition to the vote, outlining the work ahead for McCarthy.

Republican Rep. Nancy Mace from a swing district in South Carolina told reporters, “I’m still a ‘no’ on the debt ceiling.”

She called for wholesale changes to the package, including a plan to balance the budget and a restoration of the green energy tax credits that this bill rescinds. Both would be massive adjustments to the plan.

“Do the math, do the numbers, and come out with something stronger, because if this is something that the President is just going to end up vetoing and it’s a messaging bill, then we ought to have the best message, most fiscally responsible message moving forward,” Mace said.

Rep. Tim Burchett of Tennessee told CNN, “I’m a hard no. I just can’t get past $32 trillion in debt.”

Four Iowa House Republicans oppose the bill’s inclusion of a provision repealing ethanol tax breaks, according to several GOP sources. Iowa Rep. Ashley Hinson has been lobbied by GOP leaders as they try to secure her vote. Hinson declined to respond to questions about her stance as she left McCarthy’s office on Tuesday.

Some on the far right such as Rep. Matt Gaetz of Florida want to make even stricter the work requirements proposed for Medicaid beneficiaries contained in the plan even stricter. Gaetz has warned he would vote against the plan if the bill’s language isn’t changed to his liking.

But Republican leaders have – so far – resisted calls to make changes to the legislation, fearing doing so would complicate the bill’s chances for passage and force them to make a slew of other revisions.

“The plan is not to open this up again,” said one senior GOP source on Tuesday afternoon.

Meetings will continue Wednesday morning as McCarthy tries to limit GOP defections since he can afford to lose no more than four in order to pass the bill.

The Biden administration said Tuesday it would veto the House GOP’s proposed debt ceiling legislation, calling it “a reckless attempt to extract extreme concessions as a condition for the United States simply paying the bills it has already incurred.”

“The President has been clear that he will not accept such attempts at hostage-taking,” the statement from the Office and Management and Budget says. “House Republicans must take default off the table and address the debt limit without demands and conditions, just as the Congress did three times during the prior Administration.”

This story and headline have been updated to reflect additional developments.

CNN’s Annie Grayer and Haley Talbot contributed to this report.