Some of the Republican presidential candidates are going after Sen. Ted Cruz (R-Tex.), saying his birthplace could count against him in a presidential election. Here are the facts. (Sarah Parnass/The Washington Post)

Eight days ago, Donald Trump floated the idea that Ted Cruz might cause Republicans some legal headaches if he won the GOP presidential nomination because he was born in Canada -- albeit to a mother who was and is a U.S. citizen.

Today, Cruz's citizenship -- and the questions surrounding it -- is the single most dominant issue in the Republican race and has, without question, slowed Cruz's momentum in Iowa and nationally.

It's a perfect case study in both how Trump attacks and how he's able to so effectively sidetrack his opponents.

Go back to the Jan. 5 interview that the Post conducted with Trump. Here are the two paragraphs that started it all:

"Republicans are going to have to ask themselves the question: ‘Do we want a candidate who could be tied up in court for two years?’ That’d be a big problem,” Trump said when asked about the topic. “It’d be a very precarious one for Republicans because he’d be running and the courts may take a long time to make a decision. You don’t want to be running and have that kind of thing over your head.”

Trump added: “I’d hate to see something like that get in his way. But a lot of people are talking about it and I know that even some states are looking at it very strongly, the fact that he was born in Canada and he has had a double passport.”

I asked Bob Costa, who asked Trump the question, why he decided to bring it up with the real estate mogul -- given that Cruz's citizenship had been a low-level issue in the race to date.  Costa said that Trump has spoken about Cruz's background and birthplace in the past, and he wanted to see where Trump stood on the matter. He also had been hearing regularly from Trump supporters at rallies who raised the issue of Cruz's eligibility to be president.

Trump's response was in keeping with how he typically tests out attacks: He put it on "a lot of people" who were talking about the issue and insisted he wasn't entirely sure how it might all turn out but that Cruz might want to get it figured out sooner rather than later.

It soon became clear to Trump that the Cruz citizenship hit was working. John McCain, sworn enemy of Cruz, jumped on board with the "questions remain" view of Cruz's citizenship. Iowa Gov. Terry Branstad, an establishment pol through and through, threw Cruz under this bus with this quote: "When you run for president of the United States, any question is fair game. So let the people decide." Legal scholars piled on!

Like a comedian who recognizes when a bit kills, Trump made sure to not only repeatedly raise the question about Cruz's birth nation in every interview and rally but also quickly ramped up his rhetoric on the issue.

Over the weekend, at a rally in Iowa, Trump offered this gem: "You can’t have a person who’s running for office, even though Ted is very glib and he goes out and says ‘Well, I’m a natural-born citizen,’ but the point is you’re not." Then on Wednesday morning, Trump tweeted this:

In the space of a week, Trump went from "some people say this might be a problem" to "Ted Cruz is going to sink the Republican ship if he gets the nomination!!!!" And Cruz went from laughing off Trump's hit on his citizenship to issuing a formal statement insisting the matter was settled to, now, going in whole hog on attacks on Trump's "New York" values.

So, rather than Cruz spending the final weeks before the Iowa caucuses touting his social conservative credentials or railing against the "Washington Cartel" (not a thing that exists), he's playing defense on a legally fuzzy matter that simply can't be resolved to the satisfaction of Trump and his allies.

In short: Trump wins.  And confirms, again, that like him or hate him, he has an uncanny sense for what people want to hear and a knack for amplifying an attack when he grasps that it's working.