President Trump had some blunt advice for Senator Ron Johnson, the Wisconsin Republican who had been publicly excoriating his signature tax and spending bill as a deficit-driving debacle, at a tense White House meeting this month.

Be nice, the president told the businessman-turned-ultraconservative politician.

Mr. Johnson immediately toned down his vocal drumbeat of criticism against the plan. But he still looms as a significant obstacle to the sweeping domestic policy bill that Senate Republicans hope to push through next week. Not only does he declare himself adamantly opposed to the current version, he is simultaneously undercutting the party’s message that the legislation will not pile on to federal deficits.

He is, however, trying to be nicer about it.

“I’m a pretty flexible, reasonable guy,” the sometimes disputatious, three-term Republican referred to on Capitol Hill as “RonJohn” told reporters on Wednesday, drawing scoffs of disbelief. “I am!”

Mr. Johnson put his accounting background to work in a dense, 30-page report he released this week that rejects claims by the administration and Republican leaders that the combination of continued tax cuts, spending reductions in the range of $1.5 trillion over 10 years and new policies will spur growth to offset lost tax revenue. Under almost any scenario, his report says, deficits will continue to soar above $2 trillion per year, for a cumulative $20 trillion or more over the 10-year life span of the bill.

That is in line with — though substantially more favorable than — estimates by the nonpartisan Congressional Budget Office and fiscal research groups that have found that the costs of the bill would swamp any growth it could create.