Speaking to reporters ahead of an 800-person New Year's Eve bash at his Mar-a-Lago estate, a tuxedoed Trump maintained that another culprit aside from Russia could have been behind the tampering.
"It could be somebody else. And I also know things that other people don't know, and so they cannot be sure of the situation," he said.
Asked to describe what undisclosed information he knew, Trump said, "You'll find out on Tuesday or Wednesday."
Trump has scheduled an intelligence briefing midweek on the hacking, a response to President Barack Obama's announcement of new sanctions on Moscow in retaliation for the cyber-intrusion.
"I just want them to be sure, because it's a pretty serious charge, and I want them to be sure. And if you look at the weapons of mass destruction, that was a disaster, and they were wrong," Trump said, referencing failed intelligence in the lead-up to the Iraq War as a reason for skepticism.
"I think it's unfair if they don't know," he said. "And I know a lot about hacking. And hacking is a very hard thing to prove."
Trump has repeatedly cast aside a US intelligence assessment announced in early October that Moscow was behind the hacking, despite already receiving classified intelligence briefings on the matter. He's also praised Russian President Vladimir Putin, calling him smart this week for withholding reciprocal sanctions on the US.
Trump's stance has put him at odds with most congressional Republicans, who have argued for a tougher stance on Russia. Sen. John McCain, the Republican chairman of the Armed Services Committee, will convene a hearing on cyber-threats Thursday.
On Sunday morning, a Rossiya Airlines Special Flight Squadron plane believed to be carrying some expelled Russian government officials departed Dulles Airport just outside Washington. The US had ordered 35 officials and their families from the embassy in Washington and the consulate in San Francisco to leave the country by noon Sunday in response to charges that Russians hacked the presidential election and harassed US diplomatic personnel in Russia.
A State Department official told CNN early Sunday evening all of the expelled Russian diplomats have left the US. "We can confirm that the 35 Russian diplomats declared persona non grata have, along with their family members, departed the United States.," the official said.
Congressional Democrats on Sunday scoffed at Trump's assertion that Russian involvement wasn't clear.
"It's very solid," said Rep. Adam Schiff of California, the top Democrat on the House Intelligence committee, on ABC's "This Week."
Schiff called Trump's approach "dangerous," adding, "If he's going to have any credibility as president, he needs to stop talking this way. He needs to stop denigrating the intelligence."
community. He's going to rely on them.
At Mar-a-Lago Saturday, Trump expressed deep misgivings about technology during his brief exchange with reporters positioned outside his Grand Ballroom.
"If you have something really important, write it out and have it delivered by courier, the old-fashioned way, because I'll tell you what, no computer is safe," he said. "I don't care what they say, no computer is safe."
Scheduled to return to New York Sunday after two weeks at his Palm Beach mansion, Trump told reporters his New Year's resolution was to "Make America Great Again."
"I'll do that one," he said.