For decades, the GOP has been killing off its demagogue-detecting and bullshit-rejecting antibodies. Now it’s helpless.

As Donald Trump moves ever closer to their party’s nomination for president, many Republicans are trying to understand or explain what has happened. Various metaphors have been thrown around: It’s a “hostile takeover“, or a “class war“, or a “populist uprising“.

Here’s a more accurate comparison: Trump is like the opportunistic infections that attack people whose immune systems have been compromised. A healthy political party could have thrown off Trump’s candidacy with barely a sniffle, but today’s GOP is in grave danger.

Over the last few decades, the Republican Party has been systematically destroying all the habits and mores and traditions and standards that keep a political party stable and allow it to play a constructive role in governing a great republic like the United States. Those things function like antibodies: They may be invisible to the naked eye, but they head off outbreaks of all sorts of destructive nonsense.

Now they’re gone, and Donald Trump is running wild.

How did this happen? For years now, the Republican Party has increasingly been winning elections (at every level short of the presidency) by misinforming voters and appealing to their darker passions. It has pandered to believers in baseless theories like Birtherism and the gun-confiscation conspiracy, while ridiculing the scientific community’s warnings about climate change. It has claimed that racism is a thing of the past — “things have changed dramatically” John Roberts claimed while striking down the heart of the Voting Rights Act — and that the only real bigotry today is “political correctness” and discrimination against whites and Christians.

Rather than change its own plutocratic policies, the GOP has scapegoated undocumented immigrants for working-class impoverishment. (If you’ve been losing at poker and wonder if someone’s been cheating you, don’t accuse the Mexican who’s been sweeping the floor. Look at the guy with all the chips.) It has pushed self-serving economic fantasies like “tax cuts pay for themselves” and biological fantasies like the female body “shutting down” to make pregnancy-by-rape impossible. It has looked the other way while hucksters and con-men fleeced its faithful. It has struck down any traditional notions of fair play; beating Obama has been the important thing, and only wimps appeal to gentlemanly traditions and rules of decorum. (If it’s OK to yell “You lie!” during the State of the Union, what’s wrong with endorsing a shout-out that Ted Cruz is a pussy?)

In short, the GOP has devolved from the Party of Lincoln — or more recently the Party of Eisenhower — to the Party of Truthiness. (Truthiness, coined by Stephen Colbert, is the seductive notion that what your gut wants to believe must be true, independent of any facts or science or expert opinion.) The result is that the party’s base has no immune system that would reject a candidate like Trump.

All the weapons another candidate might use to take Trump down have been systematically dismantled. Are his “facts” wrong? Mitt Romney already burned that bridge in 2012. Do experts say his proposals are nonsense? There are no experts any more; if you feel a need for expert support, go invent your own experts like the Koch brothers and right-wing Christians do. Are his speeches full of racist dog-whistles? Politically correct nonsense! Racism ended in the 60s, except reverse-racism against whites. And if Republicans had to expel anybody who dog-whistled about Obama, there’d be no party left. Are there echoes of fascism in his giant rallies and cult of personality? In his celebration of real and imaginary violence against hecklers? In his fear-mongering about unpopular ethnic or religious groups? In his implication that specific policies are unnecessary, because all will follow from installing a Leader with sufficient Will? More nonsense: There is no fascism any more, unless you mean liberal fascism or Islamofascism.

With all the legitimate arguments of political discourse unavailable, other candidates were left to fight each other and wait for Trump to go away. And when Marco Rubio recently decided he finally had to take Trump on, the only weapon at hand was to tease him like a third-grader, suggesting that he wet his pants during a debate.

While many “establishment” Republicans fruitlessly look for a miracle drug to cure Trump fever without also taking down Cruz, Rubio, and half their Senate candidates, others are beginning to surrender. It’s just one election; maybe it won’t be so bad.

But this is where the compromised-immune-system analogy has something to teach: People whose immune systems have been crippled by AIDS or chemo-therapy seldom catch just one disease. Even if some massive dose of political antibiotics could flush Trump out of the Republican system, the underlying problem is still there: The Republican base cannot detect and reject hucksters. It cannot tell fact from fantasy. It values posturing and bombast over the skills necessary to govern a republic. It seeks scapegoats rather than solutions. It winks and nods at racism and white entitlement.

As long as that remains true, new Trumps will arise in 2020 and 2024, and any qualified Republican candidate offering real solutions will be defenseless against them. The Republican Party doesn’t just need to find a way to deal with Donald Trump. It needs rebuild its immune system.