WASHINGTON — President Trump vigorously defended an all-but-dead Senate health care bill on Wednesday, pressing Republicans to stay in Washington and improve the proposal or risk being tagged as supporting the current system, which he called a “big lie.”

At a lunch at the White House with Republican senators, Mr. Trump gave the latest in a series of shifting reactions to the collapse this week of the Senate’s effort to repeal and replace the Affordable Care Act, and what amounted to a belated sales pitch for a measure that Republicans have privately complained he has done little to champion.

Senator Mitch McConnell of Kentucky, the majority leader, abruptly dropped the push to replace the Affordable Care Act earlier this week and instead pressed for a vote on legislation to repeal it without an immediate alternative.

But the non-partisan Congressional Budget Office underscored why that idea quickly lost traction. The budget office said a bill to repeal without a replacement would increase the number of people without health insurance by 17 million in 2018, a figure that would jump to 27 million in 2020, and then to 32 million in 2026.

Average premiums for people buying individual health insurance policies would increase by about 25 percent next year and 50 percent in 2020. By 2026, premiums would double.

Mr. Trump has advocated a straight repeal earlier this week, arguing that…