Ted Cruz is the son of a Cuban immigrant and an American mother, born in Canada in 1970.

Is he a natural-born citizen, eligible for the presidency? Yes, of course he is. No serious person living outside the fever swamps or praying for a Hillary Clinton presidency would dispute it.

Yet somehow, the question took up several minutes of Thursday night's GOP debate on the Fox Business Network. The reason? Donald Trump has been playing with the issue, attempting to raise doubts about whether his nearest rival in the polls and possibly his most effective challenger for the nomination would even be allowed to take office as president.

A few minutes later, each member of the GOP field was asked whether he supported an immigration policy that excludes all refugees, immigrants and visitors who are Muslim. This would bar Iraqis and Afghans, amongst others, who provided vital support to Amercian forces and could face retaliation. It would punish Kurds, who are the only force effectively helping advance American policy in their region. It would include every inhabitant of several predominantly Muslim countries that have been loyal allies since at least the Cold War, not to mention more than a million Arab Israeli citizens.

None of the other candidates supported this "Muslim-free" policy, proposed by Trump, but every Republican was required to address it.

Many people have defended Trump's candidacy, arguing that he is bringing important issues to the forefront. There is truth in that. But his kooky analyses, prescriptions and simple mischeavousness are now hogging debate time, and show the extent to which he has disrupted and also tainted the Republican primary.

Trump put an exclamation point on his ignorance in an exchange about Boeing, which manufactures airplanes in Washington State and in South Carolina, just miles from where the debate was held. Trump had no idea what he was talking about when he suggested that those planes, some of which are exported to China, would soon be manufactured in China. This was just made up, and the local audience knew it. They applauded Jeb Bush, a rare event in the presidential race so far. This evidently stung Trump, who responded by testily resorting to his favorite charge that Bush is weak.

Conservatives rightly object when liberals accuse them of ignorance. But the waters are muddied by debates in which the subject matter is set by someone so proudly indifferent to the facts. Trump's trumped up birther controversy around Cruz has arisen not because it has merit but because the senator is challenging Trump's lead in the polls.

Politics, like the free market, runs on supply and demand. If Republican voters continue to provide a market for crass provocation of this sort, they will get more of it, and they will get less serious candidates and less serious debates.

Cruz decisively slapped Trump's birtherism down on the debate stage, but Trump gave as good as he got soon thereafter in a separate spat about Cruz's contemptuous dismissal of "New York values." The upshot was a debate that underlined what we already knew — that Trump, Cruz and Rubio are now the only plausible contenders for the nomination.

This also explains how difficult it is for voters to evaluate candidates clearly when there is so much chaff in the air. Which is presumably what Trump wants.