WASHINGTON — With congressional negotiations stalled over a new round of pandemic aid, President Trump has floated the idea of once acting alone to stimulate the economy before the November election.

But Mr. Trump’s initial attempts to unilaterally bolster the economy show the limitations of the president’s ability to deliver financial help without Congress.

More than a month after Mr. Trump signed an executive memorandum to defer the collection of the payroll taxes that workers pay to help fund Social Security, few companies or people are taking part. Trade groups and tax experts say they know of no large corporations that plan to stop withholding employees’ payroll taxes this fall. As a result, economic policy experts now say they expect the deferral to have little to no effect on economic growth this year.

The fizzling of the payroll plan is the most prominent example of the difficulties Mr. Trump has encountered in trying to stimulate the economy while bypassing Congress. Another of his executive actions, to repurpose disaster funds to create a temporary lift in unemployment benefits, has quickly lost steam: Federal officials told states this week that the benefits would run out after six weeks for workers.

Still, Mr. Trump has told reporters he believes he has the power to do more on his own, and might try to if Congress does not approve new stimulus. Last week, he asked Congress to free up $300 billion in unspent money from a previous rescue package to send direct checks to individuals and families, then suggested he could do so by acting alone, if he so chose.

“There’s a theory I could take the $300 billion,” the president said, after noting that he would prefer instead to get congressional authorization for the move. “It’s money that we have — money that we built up and money that we haven’t spent, and I would love to give it to the American people as a very powerful stimulus.”

Such a move could face legal challenges. Mr. Trump’s previous actions have faced logistical ones.

The payroll tax deferral was intended to goose the economy by temporarily giving workers more money in their paychecks by deferring the 6.2 percent of wages that companies normally withhold to help fund Social Security. But executives are spooked by the complexities involved in enacting the plan and the possibility that their workers — or the companies themselves — could face unexpected tax bills next year when the deferral expires.

After reviewing the details, executives at the United States Postal Service decided against enforcing it for their workers, a spokesman said Friday. Officials at Ford Motor said they also had concerns after reading the Treasury Department’s guidance, though they have yet to give employees formal guidance on the issue.

The sole large employer to enforce the plan is the federal government — and its workers have expressed anger about it. Civilian federal employee unions have urged the White House to allow workers to opt out of the deferral. Military officers have written to members of Congress with concerns that the plan could hurt enlisted women and men who are young and not yet financially savvy.

“Many of these men and women are fresh out of high school and earning the first paychecks of their lives,” an Army captain wrote to Representative Donald S. Beyer Jr., Democrat of Virginia, who represents Northern Virginia suburbs that are home to many civilian and military federal employees. “The sudden influx of money, unexplained, will likely be spent very quickly by many of them, with little regard for later consequences. Many of the soldiers within the ranks live paycheck to paycheck, and if they are not aware that all of this money must be paid back next year, it could be ruinous to their financial health.”

Not all federal employees will be forced into the deferral. The chief administration of the House told members on Friday that it would opt out of the plan, having determined “that implementing the deferral would not be in the best interests of the House or our employees.”

Mr. Trump has been fixated on the payroll tax throughout the pandemic. He pushed Congress early and often to temporarily eliminate the 15.3 percent tax on wages that helps fund Social Security and Medicare, which is split between employers and employees.

The idea flopped with Republican and Democratic lawmakers alike, despite the president’s threats to veto any economic recovery package that did not include a payroll tax cut. Instead, lawmakers voted in March to allow companies to delay payment of their half of payroll tax liabilities for the year, with the money due in installments in 2021 and 2022.

Stymied by Congress, and urged on by some of his outside advisers including the conservative economists Arthur B. Laffer and Stephen Moore, Mr. Trump signed an executive action on the payroll tax in August. He did not have the authority to eliminate the tax on his own, so instead, Mr. Trump ordered the Treasury Department to delay workers’ obligation to pay the tax through the end of the year, for employees earning up to $4,000 every two weeks.

Larry Kudlow, the director of Mr. Trump’s National Economic Council, told the Republican National Convention last month that the move “essentially gave a pay raise to 140 million working Americans.”

That did not turn out to be the case. Last month, Treasury officials issued rules clarifying that companies could choose whether to withhold employees’ payroll taxes from September through the end of the year.

Companies that did not withhold the taxes — meaning workers received a bigger paycheck — would then be responsible for paying back the money from January through April, by withholding twice the normal amount of the tax. If an employee left the company before the repayment was complete, the business would effectively find itself on the hook for the tax bill. If the employee stayed, they would be in line for what amounts to a surprise winter pay cut, with paychecks shrinking for four months to make up for the added pay in the fall.

Mr. Moore had wanted the Treasury Department to go further, and require companies to participate. But as written, tax experts said, the department’s guidance gave little incentive for large corporations to sign up — and several reasons not to.

“A lot of companies are looking at this and saying, this has too many challenges,” said Caroline L. Harris, the vice president for tax policy and economic development at the U.S. Chamber of Commerce. “I have not heard from any companies that intend to do this.”

That was not the case with the employer-side payroll tax deferral that Congress included in an early economic relief package. Some publicly traded companies, including SAIC and Staffing 360 Solutions, said in earnings calls this year that they were taking advantage of the deferral to bolster their cash flows during the economic downturn.

Some small businesses have said they will opt in and defer payroll taxes for workers, including ones owned by political supporters of Mr. Trump like an off-track betting company on Long Island, overseen by the chairman of the Nassau County Republican Party. But there were only two main reasons a private company might participate in the president’s deferral plan, said Rohit Kumar, a leader of the national tax office for PwC in Washington.

One was to provide employees with a short burst of additional cash in a difficult stretch for the economy. The other, Mr. Kumar said, would be a bet on Congress passing a bill, as Mr. Trump has requested, to cancel the unpaid tax liability for workers, turning the tax deferral into a four-month tax holiday.

But, Mr. Kumar said, “I have not advised anybody to be planning their lives around a holiday coming around next year.”

Mr. Trump said on Twitter on Thursday that he would push through such a move during a second term. But that remains far from guaranteed, given uncertainties surrounding the political makeup of Congress and the inability of lawmakers to agree on another round of stimulus aid when millions of people remain out of work.

Representative Kevin Brady of Texas, the top Republican on the Ways and Means Committee, is preparing to introduce legislation to cancel the deferred payroll tax liability. “It would be a shame if employers large and small don’t help their workers with this deferral,” he said on Thursday in a call with reporters. By approving his bill, Mr. Brady said, Democrats could “create relief for workers and certainty for businesses.”

On the call, Mr. Brady named an employer that he was certain would opt into the deferral, whether the bill passes or not: his own re-election campaign.

Jeanna Smialek contributed reporting.

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