Mr. Trump’s Twitter messages about taxes have at times sowed confusion about the direction he wants to chart for the legislation.

The president said previously that he wanted Republican negotiators to allow for a higher individual tax bracket to make sure that the bill was sufficiently progressive. The House plan would keep a top rate of 39.6 percent for millionaires and the Senate plan has a top rate of 38.5 percent for high earners. But Mr. Trump suggested on Monday that lawmakers lower the top rate to 35 percent while also giving deeper cuts to the middle class.

Last month, lawmakers were considering making changes to 401(k) retirement accounts as a way to raise revenue before Mr. Trump quashed the idea on Twitter.

Although Mr. Trump must sign the eventual legislation, Republican lawmakers have shown a willingness to break with his wishes. Mr. Trump originally called for a 15 percent corporate tax rate, but Senate Republicans appear to have settled on a 20 percent rate that will be delayed by a year. Senate Republicans also have ignored Mr. Trump’s desire to fully eliminate the estate tax and, for now, they have not addressed the special treatment of “carried interest” that gives hedge fund managers and private equity partners lower tax rates on their income.

At the beginning of the Finance Committee’s markup, which is expected to last several days, the panel’s chairman, Senator Orrin G. Hatch, Republican of Utah, offered a pre-emptive defense of the bill, both in terms of substance and process.