For most of this campaign, Donald Trump’s admiration for Vladimir Putin, the Russian president, and his willingness to act as a Kremlin apologist on issues ranging from Syria to the computer hacking of individuals and political parties have been sources of bafflement and dismay. Mr. Trump’s alarming performance at Sunday night’s debate deepened these concerns.

Mr. Trump again denied that the Russians were doing anything to manipulate the presidential election despite powerful evidence to the contrary. And he again laid bare his cockamamie and uninformed view of the bloody civil war in Syria and his refusal to acknowledge Russia’s role in making it worse.

Mr. Trump has no foreign policy experience. He has, however, received two briefings from American intelligence agencies that should have alerted him to the challenges facing the next president but apparently have not. All of which raises unsettling questions about whether the Republican nominee for the most powerful job in the world is Mr. Putin’s poodle, stubbornly naïve, totally clueless or, as some have ominously suggested, protecting undisclosed business interests in Russia.

Though allegations about Russian interference in the election have circulated for some time, the Obama administration on Friday formally accused Russia of stealing and disclosing emails from the Democratic National Committee and other institutions. When Hillary Clinton raised this, Mr. Trump came to Russia’s defense: “Maybe there is no hacking. But they always blame Russia. And the reason they blame Russia because they think they’re trying to tarnish me with Russia. I know nothing about Russia.”