A 2020 Question 100 Days Out: Will the Elections Be Free and Fair?

“If that isn’t a very disturbing, clear signal that we may have a problem, I don’t know how much more of a blinking red light you need,” he said. “Safe, secure, fair, honest elections really is the holy grail of our democracy. If we can’t trust that or don’t believe in those results, we’re no better than Russia or China or Venezuela. We’re no better than any authoritarian government.”

Mr. Trump, who has himself voted by mail before, had no precedent on which to base his remarks about mail-in voting, said Tom Ridge, who served as homeland security secretary in the George W. Bush administration.

“I regret that the president would try to undermine the legitimacy and question the outcome of the election based on some hypothetical abuse of absentee ballots” with “absolutely” no basis in history, said Mr. Ridge, a former Republican governor of Pennsylvania and a Trump critic. “When you listen to the president, you begin to wonder, is he more — is he worried about the legitimacy of the electoral process, or is he worried about losing?”

On Monday, Mr. Biden released one of his sternest warnings to date about foreign interference in the election. He has begun receiving intelligence briefings, he has said, and his advisers are on guard about potential meddling that could unfold in the homestretch of the campaign, as are some congressional Democrats who released a new warning on Monday. This week four years ago, hacked Democratic emails were released into public view. And in July 2018, special counsel Robert S. Mueller III indicted 12 Russian intelligence officers in the hacking of the Democratic National Committee and the Clinton presidential campaign.

“I am putting the Kremlin and other foreign governments on notice,” Mr. Biden said in the statement. “If elected president, I will treat foreign interference in our election as an adversarial act that significantly affects the relationship between the United States and the interfering nation’s government.”

At a fund-raiser that night, he directly linked Mr. Trump to the issue.

“He knew full well of Russian involvement in the election in ’16,” Mr. Biden said, going on to add, “He’s done nothing. He sought help. Just like he sought help to get the Ukrainians to say things about me that weren’t true and got him impeached.”

Asked for comment, a spokesman for the Republican National Committee, Steve Guest, criticized the Obama administration’s record on confronting Russia and said Mr. Biden is making a “scurrilous accusation” against Mr. Trump.