Trump veered into controversy at a news conference in Florida. He suggested that the Russians should hack into Clinton's private emails if they have not already and then release publicly those that she deleted before turning over the server to the federal Government.
No one could remember a serious candidate for president seeming to urge a foreign power to carry out espionage on the United States and at the same time call on that country to intrude on a presidential election and possibly influence the outcome. It is another example of Trump doing and saying the unthinkable and daring the Democrats and his opponents to make it cost him politically.
The controversy came on a day that Democrats were planning to use their prime-time speeches to frame the contrast between the major-party nominees and attempt to paint Trump as wholly unsuited, temperamentally and by lack of knowledge, to serve as president and commander in chief.
The lineup of speakers included the President and Kaine, Clinton's vice-presidential nominee; Vice-President Joe Biden; former Defence Secretary Leon Panetta and other former officials with national security credentials. The Democrats were already loaded with evidence to present to the American people of all the ways in which Trump has proved that he should be kept as far from the Oval Office as possible.
Trump's remarks seemed to play directly into the Democrats' hands by committing a political gaffe that they could seize on to portray him as reckless, unknowing and naive.
But that is partly because the Democrats were thinking by the conventional standards of political logic, which has never been Trump's way. Trump has defied conventions ever since he announced his candidacy and does not think that has cost him.
Trump's latest comments feed concerns that the Republican nominee is sympathetic toward Russian President Vladimir Putin and would be soft on Putin and Russian aggression if he were to become president.
He has repeatedly declined to denounce the Russian leader, and his earlier comments undermining America's commitments to Nato - most recently whether a Russian attack on one of the Baltic nations would demand a US response - have alarmed those in the foreign policy establishment.
But the foreign policy establishment is not Trump's audience. Nor is any establishment - political, economic, academic. He thumbs his nose at these elites, as do many of his most passionate followers.
The experts can express their outrage time and again at what Trump says and does about serious issues. But for Trump's supporters, it is his very recklessness and disregard for the conventions of politics that have proved so appealing.
Clinton's America sees Trump as a threat to the future of the republic and a man unsuited to occupy the presidency.
But it is Trump who continues to play havoc with the system and in so doing to set a standard of conduct that has taken this campaign into unsettling places.
