For all her struggles with poll numbers and the email investigation this year, Hillary Rodham Clinton has done one thing really well: dissuade mainstream opponents by dominating the invisible primary, the behind-the-scenes competition for elite support that often decides the nomination.

Today, her dominance in the invisible primary yielded another victory. Vice President Joe Biden’s decision to stay out of the presidential race leaves Mrs. Clinton as the only viable mainstream candidate in the race. It gives her an opportunity to unite the coalition of moderate, nonwhite and older voters who traditionally have an edge over the white progressives who now support Bernie Sanders.

Mr. Biden’s decision was informed by personal considerations, as he said Wednesday in bowing out, not just the cold calculus of building a national campaign. But the reality was that he would have struggled for the same reason that other traditional, establishment-friendly candidates decided not to run. The support from party operatives, donors and officials wasn’t quite there. The party had already decided, for Mrs. Clinton.