A lot has changed since April, when Marco Rubio announced his presidential bid. Gov. Scott Walker of Wisconsin was the top candidate of mainstream conservative activists and donors. Jeb Bush seemed like a fund-raising juggernaut with natural appeal to the party’s moderate voters, who play an underrated role in the Republican primary process. Mr. Rubio, a broadly appealing candidate but the top choice of few, looked boxed out.

Today, Mr. Rubio isn’t blocked. Instead, he has a big opening.

Mr. Walker dropped out of the race, and Mr. Bush does not look nearly as strong as it seemed he might. Mr. Bush hasn’t won many endorsements, he isn’t faring well in the polls, and his impressive fund-raising reflects a narrow base of strong support from wealthy super PAC donors, not wide support by party elites.

Can Mr. Rubio take advantage of the opening? He hasn’t quite had his moment in the media spotlight, he hasn’t made big gains in the polls, and he hasn’t earned many endorsements. But the political landscape surrounding his candidacy could not have changed much more in his favor over the last six months.

Mr. Walker’s exit is the most obvious and recent development to help broaden the path ahead. Yes, Mr. Walker had lost nearly all of his support in the polls by the time he left the race, but he still had a significant level of support from Republican elites, and Mr. Rubio has picked up several former members of Team Walker.