When did Trump become unstoppable?

Just a couple of weeks ago, many political analysts were still chattering about a contested Republican convention. Then came the Indiana primary, and the forces opposing Donald Trump seemed to unravel almost immediately. This terminal phase of the nominating season is a reminder of how much and why public opinion matters.

“Momentum” is one of the most misused terms in politics — very often it’s false signal in noisy data, especially in general elections. But primaries are different. Because they’re held sequentially, winning one state may encourage voters in subsequent states to jump on the bandwagon because they want to back a winner. Winning also provides candidates with other resources, such as money and favorable media coverage. In short, there are advantages to being the front-runner: Pluralities tend to turn into majorities.

Indeed, one theory of momentum correctly suggested that as of March 15, Republican voters would start breaking toward Trump, and that he would surge to victory.