WASHINGTON — President Trump clashed with his own party on Thursday over a stimulus package to stabilize the economy, calling for a big-spending plan of the kind envisioned by Democrats even as the top Republican leader declared that such a measure had little support within the party.

Mr. Trump declared he “would go higher” than the latest $1.8 trillion framework the White House has put forward in negotiations with Speaker Nancy Pelosi of California, faulting his own Treasury secretary for failing to offer enough money in the talks. A short time later, Senator Mitch McConnell, Republican of Kentucky and the majority leader, all but ruled out such a deal, saying senators in his party would never support a package of that magnitude.

“He’s talking about a much larger amount than I can sell to my members,” Mr. McConnell told reporters in Kentucky, a refrain he reiterated in multiple appearances across the state. Instead, he plans next week to try to advance a scaled-down $500 billion package, which is likely to fail without Democratic support.

The stark division between the top two Republicans in Washington was the latest indication of the dim pre-election prospects for enactment of a broad pandemic recovery package that economists have said is badly needed. Tens of millions of Americans have slipped into poverty, and the country’s economic recovery has faltered without federal assistance.

The public divisions also further undercut Treasury Secretary Steven Mnuchin in his attempts to reach an accord with Ms. Pelosi, who remains unmoved from the latest Democratic offer of $2.2 trillion.

Mr. Trump said in an interview with Fox Business that her proposal was so littered with Democratic priorities that his “pride” would not allow him to accept, but he also took aim at Mr. Mnuchin, saying he should “go big or go home.”

“So far he hasn’t come home with the bacon,” the president said.

His comments directly contradicted Republicans’ efforts to foist blame onto Ms. Pelosi and Democrats as the impasse has dragged on for months.

“Pelosi is holding up STIMULUS, not the Republicans!” he posted on Twitter on Thursday.

But it was Mr. Trump who abruptly pulled the plug on the talks last week, courting political risk before reversing course under pressure from alarmed Republicans.

In his most recent public concession to Ms. Pelosi, Mr. Mnuchin announced the administration would accept a key Democratic demand for carrying out a national testing strategy. “We’ll fundamentally agree with their testing language, subject to some minor issues,” he said on CNBC.

In a nearly 90-minute phone conversation with Ms. Pelosi, Mr. Mnuchin reiterated his willingness to accept Democratic language with “minor” edits and said he would provide a proposal on Friday, Drew Hammill, a spokesman for Ms. Pelosi, said in a statement summarizing the conversation. “The speaker looks forward to reviewing.”

But a number of unresolved differences remain, such as how much money to provide to state and local governments and lapsed federal unemployment benefits.

Investors, who have been following the stimulus talks closely, seemed unmoved by statements from Mr. Trump and Mr. Mnuchin on Thursday, with stocks on Wall Street dropping for a third consecutive day. And Senate Republicans, who have grown increasingly frustrated with Mr. Mnuchin’s willingness to buck their narrow proposals and capitulate to Ms. Pelosi’s demands for a sweeping relief package, were equally reticent about the prospects of a deal before Nov. 3.

“I’m proposing what we think is appropriate,” Mr. McConnell said after voting in Louisville, Ky., when asked about the targeted bill he was preparing. At later events in the state, he noted that the administration had not yet successfully reached a deal and said he felt “it was important to indicate to the American people before the election — not after — that we were not in favor of a stalemate, that we were not in favor of doing nothing.”

Mr. McConnell, who has not formally unveiled legislation before an expected vote next week, said it would be similar to a scaled-down package that Senate Republicans proposed in September, which failed to meet the 60-vote threshold.

While some Democrats have pushed for Ms. Pelosi to accept a smaller relief package, she has insisted that the toll of the pandemic merits another broad package. With the original Democratic offer costing about $3.4 trillion, the speaker has repeatedly argued that she has been more than willing to compromise with her Republican counterparts, counseling Democrats in a private phone call on Tuesday that “I don’t think our leverage has ever been greater than it is now.”

“The disdain that they have for the state and local, the contempt that they have for science by not wanting to have a national strategic plan, and again, the unfairness when it comes to America’s workers, is a tough nut to crack.,” Ms. Pelosi said in an interview on MSNBC on Wednesday. “Still and all, we want to try and find our common ground.”

During the phone call, Ms. Pelosi raised Mr. McConnell’s comments in Kentucky and Mr. Mnuchin “indicated that the president would weigh in with Leader McConnell should an agreement be reached,” Mr. Hammill said.

In the CNBC interview, Mr. Mnuchin did not directly address the lack of support for a bill among Republicans, though he acknowledged that they preferred a more targeted relief bill that would provide a fraction of the new funding he and Ms. and Pelosi were discussing.

And after conceding on Wednesday that it would be difficult to pass and enact a deal in the next three weeks, Mr. Mnuchin continued to push for Congress to give him the authority to repurpose about $300 billion in unused relief money from the $2.2 trillion legislation that was passed in March.

Ms. Pelosi and Democrats have largely resisted voting on stand-alone bills.