Mr. Trump’s tweet concerned one of those accounting maneuvers, which would have allowed Republicans to effectively borrow tax revenues from the future to offset some rate cuts today. Reducing 401(k) contribution limits would force retirement savers to pay more in taxes today, as they sock away money, but less in the future, when they began withdrawing retirement funds tax-free.

Republicans had not decided whether to include a reduced cap on contributions in their final version of the tax bill even before Mr. Trump’s tweet.

Details of the Republicans’ tax bill have been closely held, and they would not comment on Friday about possible changes to 401(k) policies. It was not clear from Mr. Trump’s Twitter post on Monday whether he meant that he would not support a bill including alterations to 401(k) limits or that he knew the Republicans’ draft bill did not include such changes. Several sources said last week that such changes were under consideration as House Republicans prepare to release a tax bill in the coming weeks.

Democrats and other critics of Mr. Trump’s tax plan have said it would not help middle-class Americans, despite White House and Republican promises. “Tax cuts for corporations and the wealthiest Americans should not be paid for by increasing taxes on middle class Americans saving for retirement,” a group of Democratic senators, led by Senator Sherrod Brown of Ohio, wrote to the administration in September.

Any plan to cap 401(k) savings could bolster those arguments.

Republicans are discussing proposals that would potentially cap worker contributions at $2,400 annually for 401(k) retirement accounts, lobbyists and consultants have said. Currently, workers can put away $18,000 a year in tax-deferred plans; workers who are over 50 years old can save up to $24,000.