WASHINGTON — President Trump’s personal lawyer on Monday urged the Treasury Department not to hand over Mr. Trump’s tax returns to House Democrats, warning that releasing the documents to lawmakers he accused of having a “radical view of unchecked congressional power” would turn the Internal Revenue Service into a political weapon.

It was the second such letter written on behalf of Mr. Trump since Representative Richard E. Neal, the Democratic chairman of the House Ways and Means Committee, formally requested six years of the president’s personal and business tax returns earlier this month. Mr. Neal on Saturday gave the Internal Revenue Service until April 23 to provide him with the tax returns after Steven Mnuchin, the Treasury secretary, said last week that he could not meet an earlier deadline because he needed to study the lawfulness of the request.

The fight over Mr. Trump’s tax returns is expected to turn into a protracted legal battle that will likely make its way to the Supreme Court. Mr. Trump, who declined to provide his tax returns while running for president, continues to cite an ongoing audit as the reason he cannot release the documents, despite no rule or law that prevents him from doing so.

Mr. Neal used an obscure provision of the tax code to request the returns, which he said his committee needs in order to evaluate the policy of automatic audits of presidential tax returns. Mr. Trump and his defenders argue that the request is being driven by politics and represents presidential harassment.