Donald Trump pledged his allegiance to the Republican Party yesterday, ruling out an independent bid. That appears to be a coup for the party: had the property tycoon decided to run alone, he could have drawn enough Republican voters to end the party’s chances of winning the White House in 2016. But Mr Trump’s promise also brings risks for the Republicans. The Economist interviewed him for this week’s cover story (available, with a recording, at Economist.com). Just in case there were any doubt on the matter, the interview makes clear that it would be no joke to have the rambling billionaire anywhere near the nuclear button. Granted, it is hard to see that happening: there are not 66m Americans (Barack Obama’s total in 2012) who would vote for Mr Trump. But he could do real damage to the Republican Party along the way.