Why be Trump’s running mate?

Over the last year, stories have run about how Gov. Doug Burgum, Sen. Tim Scott, Sen. J.D. Vance, former Rep. Tulsi Gabbard and Sen. Tom Cotton, among others, "leads the pack for Trump veep pick." A recent headline declared the likeliest candidate now is Sen. Marco Rubio of Florida.

No one knows for sure, of course; Donald Trump said on Saturday that he has made his mind up, but he says a lot of things. The presumptive Republican nominee has said repeatedly that the i...