It will be as much a make-or-break moment for McConnell as for Trump. The Senate leader has so far been able to fly below the radar on health care as House Republicans worked through their disagreements before ultimately passing a bill. If he cannot do the same, he is likely to be blamed for the collapse of the effort to fulfill a signature GOP campaign promise.

McConnell is cool and deliberative while Trump is hot and impetuous. But they have privately developed what people close to them say is a respectful relationship.

In the 75-year-old majority leader, Trump, 70, sees a senior player in navigating the ways of Washington, in both age and experience. He views him as someone on his level — or at least more on his level than many other Republicans, including House Speaker Paul D. Ryan (R-Wis.).

In some regards, McConnell has become a tutor to Trump. The two men speak regularly, with McConnell initiating some calls to guide the novice president.