Donald Trump is a symptom, not a cause, of the intellectual rot within the Republican Party. Put another way, he’s what the doctors would call an “opportunistic infection,” a malady that couldn’t take hold in a healthy host but that thrives in a host with an already weakened immune system.

Marco Rubio, the great slight hope of the waning Republican establishment, offered confirmation of that diagnosis this week in his remarks to an American Legion group in New Hampshire. “It’s now abundantly clear,” Rubio said. “Barack Obama has deliberately weakened America.”

Think about that: Rubio wasn’t arguing that Obama’s policies had been flawed, leaving the country weaker as a result. He was claiming intent. He was going to motive. Rubio explicitly argued that the twice-elected president of the United States, our commander in chief, set out to deliberately weaken the country because he believes it is a force for evil in the world.

We’ve always had people who said or implied such things. Cynthia McKinney, the former Georgia congresswoman, believed that George W. Bush deliberately allowed the attacks of 9/11. A couple of generations back, some muttered that FDR had exposed our fleet at Pearl Harbor to Japanese attack so he could draw us into World War II, or that Ike Eisenhower was a “conscious, dedicated agent of the communist conspiracy,” as the Birchers used to put it. But they have typically been fringe figures, people whose irresponsible rhetoric ensured that they were kept far from the levers of power.

Rubio, on the other hand, is a leading presidential candidate, someone viewed as the probable champion of his party’s mainstream. As his example demonstrates, once your party deems it acceptable to spread such nonsense by dog-whistle, you have no standing to complain when someone like Trump barges in and does so via bullhorn.

It’s also important to recognize just how corrosive such allegations are not just to the Republican Party but to democratic self-governance. In the end, our system is founded on mutual trust; without it, the Constitution is a dead letter. Republicans and Democrats will always fight and bicker, but as long as we accept the errors of the other party as errors made in good faith, we can also accept our victories and defeats and go on as fellow Americans to argue again tomorrow.

However, once one party succumbs to the notion that the other is guilty of treachery, of actively trying to undermine the whole country, as Rubio implies, the system cannot function. In such an environment, the compromise that fuels a constitutional republic becomes redefined as collaboration with the enemy. Faith in the system disappears; radical responses are legitimized and gunmen start taking over government buildings.

Once unleashed, such an attitude is hard to contain. The old-school elements within the Republican Party that still appreciate its danger — Jeb Bush, John Kasich, Lindsey Graham, etc. — have lost the will or capability to corral it, and the likes of Rubio have surrendered to it. If it is to be consigned once again to the fringes, where it belongs, the American voter will have to resoundingly reject it.

I hope that’s what happens. But we’ll see.