We’ve previously doubted whether Mr. Trump really intends to spend large sums of his own money chasing the White House or whether he’s just playing us. His first TV ad began running this week and famously includes an inaccuracy. A scene meant to suggest illegal immigrants storming the U.S. border actually shows North Africans trying to cross into a Spanish enclave on the Moroccan coast.

The $2 million Mr. Trump reportedly has committed to ad spending likely is not coming from his pockets—the sum represents about half the “unsolicited donations” he reported in the third quarter. His first TV ad might have been an opportunity to broaden his pitch. Everybody already knows about his proposed border wall with Mexico. His protectionist-nativist shtick has received saturation coverage from the media.

As the New York Times reports, Mr. Trump’s fans are disproportionately Democrats who lean Republican and have a low propensity to vote. Even so, his ad doubles down on these fans. Mr. Trump is focused on niche deepening, not niche broadening, likely because free media amplification makes it remarkably cheap for him to do so, and because he is whipping up a fan base that he knows will be useful to him beyond 2016 politics.