For months, all the political oxygen in Staten Island was being consumed by the Republican showdown between Representative Dan Donovan and Michael Grimm, the former congressman and convicted tax felon.

But all the while, Max Rose, a 31-year-old Army veteran and health care executive, was waiting in the wings — if waiting can mean corralling votes with the tenacity of a sheepdog, out-raising and out-canvassing his five Democratic challengers.

On Tuesday, his steady crusade paid off, with Mr. Rose winning 65 percent of the vote in the primary in the 11th Congressional District, which takes in all of Staten Island and a chunk of South Brooklyn.

Mr. Rose, who happily answers to progressive, moderate, liberal — even “blue dog” — refuses to concede that the borough is inherently conservative, and thus unwinnable. President Trump won Staten Island by 17 points, but Democrats outnumber Republicans in the district by a five-to-three ratio.