DETROIT — Donald Trump won the South Carolina primary across the board, but he did particularly well with the 10 percent of voters who named immigration as the nation's top issue. In addition, some who named other issues — the economy, national security — were undoubtedly also concerned about immigration, and Trump's hard line likely helped him with them, too.

Which is why people who follow immigration closely were stunned Thursday night when Trump, at the Fox News debate here in Detroit, announced that he has changed his position on one key element of the immigration debate — the use of H-1B visas to bring skilled foreign workers into the United States.

In the distant past — say, yesterday — Trump focused on abuses in the system, in which some big companies have been caught using H-1Bs to bring in foreign workers, force American employees to train their own replacements, and then pay the foreign worker less than the American had made — all to do mostly routine jobs in the tech industry.

At his recent rally in Alabama — the one in which Trump received the endorsement of Sen. Jeff Sessions, Congress' strongest voice against expanding the troubled H-1B program — Trump also won the endorsement of some American workers who were victims of H-1B abuse at Disney.

"The fact is that Americans are losing their jobs to foreigners," one of the laid-off workers told the crowd. "I believe Mr. Trump is for Americans first."

In Detroit, Fox News' Megyn Kelly pointed out that Trump's campaign website has a strong statement against increasing the number of H-1Bs, saying it would "decimate American workers," and yet in one debate Trump spoke favorably of the program. "So, which is it?" Kelly asked.

"I'm changing," Trump said. "I'm changing. We need highly skilled people in this country, and if we can't do it, we'll get them in. But, and we do need in Silicon Valley, we absolutely have to have."

"So, we do need highly skilled," Trump continued, "and one of the biggest problems we have is people go to the best colleges. They'll go to Harvard, they'll go to Stanford, they'll go to Wharton, as soon as they're finished they'll get shoved out. They want to stay in this country. They want to stay here desperately, they're not able to stay here. For that purpose, we absolutely have to be able to keep the brain power in this country.

"So you are abandoning the position on your website?" asked Kelly.

"I'm changing it," Trump said, "and I'm softening the position because we have to have talented people in this country."

Trump's turnaround sent a jolt through the group of policy wonks and activists who have opposed Gang of Eight-style comprehensive immigration reform. "I've heard from enough tech workers displaced by H-1Bs that Trump's apparent answer very dispiriting," tweeted the writer Mickey Kaus. "Clarification?"

Mark Krikorian, head of the Center for Immigration Studies, which favors reducing levels of immigration into the U.S., was not impressed. "[Trump] made clear in October he didn't believe what's in his immigration paper about skilled immigration," Krikorian told me by email after the debate, "and at the last debate he showed he buys the 'jobs Americans won't do' line on unskilled workers too."

"So will he 'clarify' his 'I'm softening' comment tomorrow, like he did after the October debate?" Krikorian continued. "His embrace of foreign tech workers is particularly shocking given that just days ago he featured American workers replaced by Disney at one of his rallies."

Even as Krikorian was typing his email to me, Trump was at work doing just what Krikorian predicted. "Megyn Kelly asked about highly-skilled immigration," Trump said in a clarification statement sent to reporters about an hour after the debate ended. "The H-1B program is neither high-skilled nor immigration: these are temporary foreign workers, imported from abroad, for the explicit purpose of substituting for American workers at lower pay. I remain totally committed to eliminating rampant, widespread H-1B abuse and ending outrageous practices such as those that occurred at Disney in Florida when Americans were forced to train their foreign replacements. I will end forever the use of the H-1B as a cheap labor program, and institute an absolute requirement to hire American workers first for every visa and immigration program. No exceptions."

It would be hard to imagine a quicker or more complete flip-flop.

Sen. Marco Rubio, co-author of the Gang of Eight bill, wants to increase the number of H-1Bs; he's a co-sponsor of another bill to do just that. After the debate, top Rubio campaign aide Todd Harris was happy to entertain questions about Trump's changing positions.

"First of all, I'm not surprised that he so easily took what yesterday was something that he had said was a core principle and threw it out the window, because the fact is that Donald Trump has no core principles," Harris told me. "One of the hallmarks of a con man is to say whatever it is that you need to say in order to fool somebody, and he obviously feels that what he needs to say to fool the people now is different than what it was yesterday."

I asked whether Sen. Rubio is happy to have Trump join him in advocating for H-1Bs. "Sen. Rubio supports the H-1B program," Harris said. "The problem is we have no idea where Donald Trump is going to be tomorrow on this issue or frankly, any other issue."

A short time later, Rubio himself issued a statement noting that in the debate Trump "finally took an actual position, but as soon as the debate was over, his handlers made him reverse himself."

"The Republican nominee," Rubio said, "cannot be somebody who is totally clueless on so many issues, including his signature issue."