Donald Trump’s low approval ratings have attracted a crush of Democrats vying to make him a one-term president. But he might have an unlikely ally in his re-election bid: Democrats’ mess of a primary system.

The most valuable commodity in a zillion-way presidential primary is attention: In 2016, much of Mr. Trump’s primary success owed to saying and doing so many outrageous things that none of the other 16 Republican candidates could compete for media oxygen against the more than $2 billion in free media his relentless antics generated.

But an overlooked key to Mr. Trump’s 2016 upset was the Republican primary system, which winnowed the 17-candidate field quickly and gave Mr. Trump a head start at jackhammering away at Hillary Clinton.