Once upon a time, Sen. John McCain was one of the few nationally known Republicans whose M.O. did not include bashing illegal immigrants for possible electoral gain. Those days are long gone. Spooked by the ugly xenophobia rampant in the GOP’s base, McCain has joined his party’s mainstream in race-baiting, immigrant-hating rhetoric, much of it utterly unsubstantiated. He gave a prime example this weekend, citing “substantial evidence” that some of the wildfires that have ravaged hundreds of thousands of acres in Arizona this month “have been caused by people who have crossed our border illegally.”

The truth is, there is no such “substantial evidence.” And in fact, there is not even a shred of evidence. In trying to defend his statement today, McCain made that very point himself, issuing an explanatory memo that, on close reading, makes it clear that his statement was based on absolutely nothing. Except, of course, his own need to prove his bona fides as an anti-illegal-immigrant firebrand.

“For years,” said the statement, attributed to McCain’s communications director, “federal, state and local officials have stated that smugglers and illegal immigrants have caused fires on our southern border.”

Ah, the old “for years” dodge. The news release goes on to quote this and that official (one from 2006) as offering a variety of explanations for fires past and present, including the possibility that some may have been set as diversions by illegal border-crossers.

And the “substantial evidence” that the current, hugely destructive fires were set by illegal immigrants? Well, not so much. On Sunday, a U.S. Forest Service official told CNN that the blaze was probably caused by an “escaped campfire” — and there was no reason to think illegal immigrants were behind it.

Facts are sticky things, particularly for McCain where it concerns the border. For months now, he has been insisting that sweeping immigration reform (of the sort he once championed, back before his party turned on him because of it) is impossible until the border is secure. He said the same thing this weekend, preposterously suggesting that the way to prevent wildfires in Arizona was to tighten up the border.

In fact, as McCain has been told repeatedly by the Border Patrol and other agencies, the frontier with Mexico is now more secure and less porous than it has been in decades, thanks to huge additions of agents and resources initiated by President Bush and sustained by President Obama. Illegal crossings are down dramatically, and in many sectors, bored agents patrol terrain where crossers are few and far between. The reality is that illegal crossings, deterred by the massive buildup of agents on the border as well as the economic slump, have been cut in half since 2005.

Those are the facts. But McCain hates to be bothered by the facts. For him, it’s politically expedient, and safer, to hew to the Republican orthodoxy, which prefers to paint the border as out of control. But by appeasing the xenophobes in the Republican base, McCain and Co. are fanning the flames of racial hatred in the Southwest and beyond. And there really is substantial evidence of that.