Many observers were mystified when Donald Trump attacked New Mexico Republican Gov. Susana Martinez. But the story was really very simple: Martinez hit Trump, so Trump hit back. Especially now that Trump is the GOP's presumptive nominee, he attempted to make an example of a Republican who won't get with the program. It might work, or it might not, but from Trump's perspective it's the tactic he used to beat 15 rivals for the GOP nomination.

The Trump-Martinez bewilderment focused on four factors: Martinez is Hispanic, she's a woman, she's a Republican (head of the Republican Governors Association), and she's popular. "I think it sent all the wrong signals," said Newt Gingrich, who has generally been pro-Trump. "You particularly don't want to see your candidate who needs to...get stronger with Latinos, and stronger with women, attack a Latina woman Republican governor."

"[Trump] has a problem with women, Hispanics, and Republicans," said George Will on Fox News Sunday, "so he attacks a Republican Hispanic woman governor."

Trump ignored all those concerns. Also on Fox, campaign manager Corey Lewandowski explained that Trump's remarks about Martinez at a May 24 rally in Albuquerque — Trump said, "The governor has to do a better job. She's not doing the job" — was simply a continuation of Trump's habit of discussing economic problems wherever he travels. "Outlining the economy of a specific location is something that he has done across the country, everywhere we've gone, because we need to highlight the problems that we have and what we're going to do to fix those problems," Lewandowski said.

It's true. Trump has pointed to economic problems in state after state. But he has no more Republican rivals to defeat now, and he is trying to consolidate GOP support. And there are those problems with Hispanic and women voters. (Indeed, many Republican criticisms of Trump's Albuquerque speech could be boiled down to "But, but, she's...Hispanic!") So why go on the offensive?

To Trump, there was something more important at work. "[Martinez] continues to attack him publicly and privately," one person in TrumpWorld told me recently. Trump has made a principle of hitting back harder than he is hit. And he has been so effective that many Republicans, elected and not, have decided the smart thing is to refrain from taking on Trump, even if they oppose him.

Not Martinez. In mid-April, the New Mexico governor issued a "remarkably strong rebuke" to Trump, in the words of a Washington Post report, when Martinez spoke to a GOP fundraiser at the home of David Koch in Palm Beach, Florida. Martinez, according to the Post, "did not mince words. She told the crowd of about 60 wealthy GOP backers that, as a Latina, she was offended by Trump's language about immigrants. Noting her years working as a prosecutor on the Mexican border and now as a border-state governor, Martinez said Trump's plan to build a wall and force Mexico to pay for it was unrealistic and irresponsible, according to multiple people in attendance."

Team Trump believes Martinez has continued to criticize him in private since those remarks. And when Trump traveled to Albuquerque, after having clinched the Republican nomination, Martinez told reporters she was "really busy" and did not have time to attend.

So Trump slammed Martinez for, among other things, the state of the economy and a rise in food stamp usage in New Mexico. "We have got to get your governor to get going," he said. "She's got to do a better job."

"There was no attack on a Latino or a woman governor," Lewandowski said on Fox. "What this was, was laying out the economic perspective of what the state of New Mexico was doing, and he's saying we need to do a better job."

It was that — plus a threat to Martinez to get in line.

Trump confirmed as much at his news conference Tuesday. Asked why he went after Martinez, Trump said, "She was not nice. And I was fine — just a little bit of a jab. But she wasn't nice, and you think I'm going to change? I'm not changing, including with her."

Trump continued: "If I have a Republican that's not on my side, why should I be particularly nice to that person? I'm not going to go after her like I would Hillary or Crazy Bernie, but you know what? Why should I be nice to that person? If I have a person that's not going to support me, I have no obligation. Politically, I may be right, I may be wrong, but that's who I am. I'm a very honest person. If somebody is going to say a little bit negative or a lot negative about me, and if they happen to be a Republican, I may choose to hit them back. Not always, but I may choose to hit them back."

As for Martinez's popularity — yes, she is popular with Republicans in New Mexico; a recent PPP poll found she had a 67 percent job approval rating with GOP voters in her state. That is popular, but about 15 points lower than Scott Walker's approval rating in Wisconsin when Trump attacked him during that state's primary campaign — a move seen as a disastrous mistake after Trump's loss.

Among New Mexico independents, Martinez's job approval rating is just 38 percent. Among Democrats, it's 32 percent, for an overall job approval rating of 47 percent.

As far as the general election is concerned, New Mexico isn't a bright spot for the GOP. Barack Obama won the state by ten points in 2012 and 15 points in 2008. Indeed, New Mexico has gone for Democrats in five of the six presidential elections dating back to 1992. (George W. Bush won the state in 2004.)

In 2012, according to exit polls, 37 percent of those who voted for president in New Mexico were Hispanic. They voted for Obama over Mitt Romney 65 percent to 29 percent.

Perhaps Martinez will change her tone after Trump wins New Mexico's primary on June 7. Perhaps not. As far as Trump is concerned, attacking Martinez, to the (unknown) degree that it angers Hispanic voters in general, could affect his fortunes far outside the boundaries of New Mexico. But Trump apparently felt it was more important now to crack down on Republican resistance, at least on GOP politicos who openly attack him, than it was to ignore Martinez's opposition.

It's not the way things are done these days. But the person in Trump World looked back to a time when powerful party bosses would occasionally make an example of an off-the-reservation official just to inspire others to stay in line. That's still necessary, the person said, and that's what Donald Trump is doing.