Mr. Trump has weighed in as Senate Republicans are struggling to decide how much of the Affordable Care Act should be eradicated and how much should be retained. In the Senate this week, Republicans veered from their original approach and said they were discussing whether to keep a tax imposed by the Affordable Care Act on the investment income of the most affluent Americans. The revenue could be used to increase insurance subsidies for lower-income people.

Subsidies in the Senate bill were already beginning to look like those in the Affordable Care Act, which are tied to a person’s income and local insurance costs. However, the Senate subsidies are less generous than those under the current law.

The repeal bills written by House and Senate Republicans would provide tens of billions of dollars in assistance to health insurance companies to help stabilize insurance markets and hold down premiums. Many of the same Republicans attacked such payments, when made by the Obama administration, as a bailout for the insurance industry.

But as senators tried to come to agreement, Mr. Trump effectively added a distraction. A clean repeal of the Affordable Care Act would face huge political obstacles in the Senate if it was not accompanied by legislation to provide health coverage in some other manner. Republican senators are already faced with pleas from constituents who want the health law to remain in place. If they approve a repeal-only measure, they will face enormous pressure to explain what comes next.

Mr. Trump’s remarks on Twitter offered a reminder to congressional Republicans of how inconsistent the president has been as a partner. In May, he cheered passage of a House plan to repeal and replace the Affordable Care Act, only to later denounce the House bill as “mean.”

Senator Patty Murray of Washington, the top Democrat on the Senate health committee, said that with his tweet, Mr. Trump was “sending a signal to Senate Republicans that they can count on him to support their back-room deals to jam their bill through just about as much as House Republicans could count on him before he called their bill ‘mean.’”

Mr. McConnell is also facing pressure from restive conservatives inside and outside the Senate who are dissatisfied with his repeal bill. But placating them risks driving away other members of his caucus.