Something strange has happened to the Republican presidential field since Donald Trump joined it a month ago: Mr. Trump and Jeb Bush are rising. Everyone else is falling in the polls, or seems stuck in place. The numbers suggest Mr. Trump is shaking up the GOP primary electorate in a meaningful way. He seems to be taking support from the most conservative and anti-Washington rivals in the field, particularly Mike Huckabee, Scott Walker, Ted Cruz and Rand Paul. Mr. Bush’s rise in the polls suggests that his base of support has little overlap with Mr. Trump’s, and he therefore may...