Since when did Arizona become a swing state? Since Donald Trump became the Republican nominee for president.

To get a full measure of the recent free-fall of the Donald’s campaign, look no further than the home of former GOP White House hopeful John McCain. At the end of July, the psephology site had odds of Trump winning the Grand Canyon State at a robust 75 percent. A little over a week later, the odds have shifted dramatically, and are now in Clinton's favor, 53 percent to 46.

These are betting odds, and not to be confused with the polls themselves or the weighted-average projection of what the vote breakdown will be. But even by those measures, the news is spectacularly bad for Trump. At the end of July, he was up six points over Hillary Clinton. Now he's behind by a fraction of a point.

I'll allow that the state is not as deep-red as it used to be. As Arizona has grown (its population has nearly tripled since 1980), it has moved away from the Goldwater conservatism I knew in my Phoenix youth. Many are the Democrats who have escaped failing East- and West-coast localities for the dynamic livability of the Southwest. Alas, they have brought with them the liberal voting habits that made California and New Jersey places to flee in the first place. Arizona also has a growing population of Hispanic voters who, Democratic-leaning to begin with, have been given a shove by Trump.

But that said, Arizona is still a red state. Since 1952, its Electoral College votes have gone to the Republican candidate in every presidential year except for 1996, when Ross Perot siphoned off enough deciding voters to give the state to Bill Clinton. Four years ago, Mitt Romney took the state by a nine-point margin. If Trump has to contest Arizona, he's done.

For starters, it tells you about the overall electorate: If he's not ahead in Arizona, Donald can't possibly be ahead in the traditional swing states of Ohio, Pennsylvania, and Florida. But beyond that, Trump can't afford to spend time in a rearguard effort to keep Arizona from slipping away. No doubt if the Donald campaigns in Arizona, he will be met by huge numbers of supporters. But they were going to vote for him anyway. It's the undecided purple-state voters he needs, and any day he spends in Scottsdale is a day he isn't spending in Cincinnati or Tampa.

Which is why the Clinton campaign is floating word that they are going to bulk up their operations in Arizona. Even if Hillary isn't likely to win the state, by contesting it she harasses Trump's flank and keeps him from concentrating on the key states that could determine the election's outcome.

The Trump team will no doubt point to polls in Arizona that offer better news for their candidate—the most recent one of the state, conducted by CBS News and YouGov, still shows Arizona leaning to Trump. But keep an eye on what the candidates do in Arizona: If Trump spends much time there, it will mean that his people believe the dire polls have it right.

And if Trump does have to spend much time in Arizona, he might as well keep a lookout for distressed golf course properties there. He'll have plenty of time to devote to his real-estate business come November.