It’s no surprise that when President Donald Trump shot back at U.S. Sen. John McCain, R-Ariz., last week, he failed to engage the substance of McCain’s argument.

“Yeah, well I hear it. And people have to be careful because at some point I fight back,” Trump said in his usual playground style. “I’m being very nice. I’m being very, very nice. But at some point I fight back, and it won’t be pretty.”

But make no mistake: The challenge that McCain threw down a week ago, and that former President George W. Bush joined Thursday, is very much about an idea. It is about whether democracy, on the defensive around the world, can recover its confidence. That in turn depends in large part on whether the United States will honor what McCain called “the obligations of international leadership and our duty to remain ‘the last best hope of earth.’ ”

It’s not easy to fathom the extent to which authoritarianism has been gaining ground, and liberty slipping, around the globe for the past decade, after years of movement in a more positive direction. On the same day that McCain spoke, as he accepted the 2017 Liberty Medal from the National Constitution Center, I happened to be at a seminar at the Hoover Institution in Palo Alto, Calif., that brought the trend into sharp relief.

Larry Diamond, a Hoover and Stanford University democracy expert, said research shows more clearly than ever that democracy is associated with economic growth, personal freedom and civil liberties, and transparency and reduced corruption. He said polls show that demand for democracy remains high, including in regions of Africa where many governments fail to deliver.

Yet, Diamond said, the world is experiencing a “deepening democratic recession” characterized by fewer nations living freely, as measured annually by Freedom House; by increasing power projection from Russia and China; by the breakdown of democracy from Turkey to Thailand to Hungary; and by a “wave of illiberal populism” and a “decay of democratic values and self-confidence in the U.S. and Europe.”

Diamond recently received a poignant message from a civic leader in Uganda, where President Yoweri Museveni, in power since 1986, is amending the constitution so he can extend his rule still longer.

“Members of the armed forces entered parliament last week and beat up MPs who were protesting the bill to lift the age limit,” the activist wrote. “It appears to me the whole region is in a steep democratic recession, partly because of the loud silence from their western allies.

“In the past, the state was a little reluctant to be this brute and violent and had some measure of shame,” the message continued. “It is all gone.”

You can debate the causes of this democratic decay, which obviously did not begin with Trump. Blame Bush’s invasion and bungled occupation of Iraq; President Barack Obama’s abandoning Syria to its grisly fate; or deeper, underlying trends of wage stagnation, economic inequality, fracturing social media.

But Trump’s admiration for strongmen and contempt for democratic values has accelerated the trend and shocked much of the world. Kori Schake, a Republican defense expert and Hoover fellow, said at the same seminar that the global anxiety today reminds her of what she was hearing from allies after the 9/11 attacks: People “are a little bit scared that America is becoming a different place than they thought they knew.”

Many Americans are worried about that, too. When I repeated Schake’s comment to Michael McFaul, a Hoover and Stanford scholar who served as Obama’s ambassador to Russia, he said he is “optimistic about our ability for renewal as a democratic society.”

But when it comes to global leadership, McFaul was less of an optimist. “The retrenchment period began under Obama, let’s be clear,” he said. “Trump is a more extreme version. That I fear may be permanent.”

The good news is that the United States has hardly tried to counter the authoritarian offensive — which means that, if we stirred ourselves to act, we might yet shift the momentum. That is clearly what the old lions of the Republican Party are hoping for.

“For more than 70 years, the presidents of both parties believed that American security and prosperity were directly tied to the success of freedom in the world,” said Bush. “And they knew that the success depended, in large part, on U.S. leadership.”

“We will not thrive in a world where our leadership and ideals are absent,” said McCain. “We wouldn’t deserve to.”

The sad news is how few Republican leaders can bring themselves to defend traditional American ideals in the same way. Unless they do, the democratic recession will be increasingly likely to shift from cyclical phenomenon to the onset of something much scarier.