But when he assembled a group of senators to cobble together a health care bill last month, he seemed to go out of his way to exclude some of the most knowledgeable members and moderate voices on health care, like Senators Bill Cassidy of Louisiana, a doctor, and Susan Collins of Maine, an insurance expert and one of the few women in the Senate Republican conference. Views outside of Mr. McConnell’s on health care did not receive extensive consideration.

When Republicans from states that had expanded their Medicaid programs quickly found themselves at odds with more conservative members who wanted a large rollback of Medicaid, Mr. McConnell did little to allay those worries. Conservatives generally wanted to rein in costs while moderate members wanted to increase spending, particularly in states where health care costs are high and opioid addiction is escalating.

On those key issues, Mr. McConnell put his legislative thumb on the scale in favor of conservatives, quickly alienating many senators from states that had expanded Medicaid, such as Senator Lisa Murkowski of Alaska and Ms. Collins, who became an early and vocal opponent of the bill.