Administration officials have expressed frustration that there was not a better explanation of the three-phase approach described by the House Republican leadership after the bill was unveiled, lamenting the resulting confusion…

For the president and the speaker, passage of the bill is about more than the health care debate; it is a matter of demonstrating that major legislation — with the weight of the White House behind it — can sweep through a Republican Congress.

On Capitol Hill, Republicans are already confronting concerns that a stumble on the first major agenda item would imperil future efforts on tax reform and a border wall.

“The legislative window closes a lot sooner than people imagine,” said Peter Wehner, a former director of the White House Office of Strategic Initiatives under President George W. Bush, who has known Mr. Ryan for two decades. “It’s open the first year, and you better get things done. If you win, that builds on itself. And if you lose, that builds on itself.”