Senate Republicans, eager for a major legislative achievement after the Affordable Care Act debacle, have generally been enthusiastic about the tax overhaul. But given the party’s slim 52-to-48 majority in the Senate, the reluctance of even a handful of Republican senators has left open the possibility that the tax bill will be further changed or face failure.

Beyond Mr. Johnson, the support of several other Republican senators was not assured, including the three who killed the party’s effort to repeal the health law this summer: Ms. Collins, John McCain of Arizona and Lisa Murkowski of Alaska.

“I want to see the whole package before I make a decision,” Mr. McCain said.

On Wednesday, Ms. Collins expressed concern that middle-income consumers could see their tax cut erased by an increase in health insurance premiums caused by the repeal of the requirement that most people have insurance, known as the individual mandate.

Other Republican senators, such as Mr. Corker and Jeff Flake of Arizona, are concerned about the national debt, which topped $20 trillion even before consideration of a tax plan that could add an additional $1.5 trillion over a decade.

As the Senate Finance Committee continued its formal drafting of the bill, Democrats attacked Republicans for inserting the repeal of the individual mandate and for imposing a 2025 expiration date for individual tax cuts, even as they would make the corporate tax cut permanent.

“This bill seems to get worse by the hour,” said Senator Ron Wyden of Oregon, the top Democrat on the Senate Finance Committee. “This is not just another garden variety attack on the Affordable Care Act; this is repeal of that law.”

Repealing the health law’s individual mandate would allow Republicans to save more than $300 billion over 10 years, giving them more room to cut taxes. According to the Congressional Budget Office, it would also lead to a reduction of 13 million in the number of people with health coverage, and average health insurance premiums on the individual market would rise by about 10 percent.