It's been clearly assessed by the American intelligence and by the Robert Mueller report that Russia did hack the Democratic party e-mails in 2016 and operated more interference on the election.if Russia wasn't involved then
Trump also openly invited Russian interference, by saying "Russia, if you are listening, I hope you can find those 30,000 e-mails that are missing", and a few hours later these were released.
Trump is now also famous for creating quid pro quos: offering something or threatening something in exchange for favors. He did it with Ukraine, he has done it recently with the New York State to prevent them to investigate his tax returns, and I am not surprised at all if he offered pardon to Assange if he denied the Russian interference, only to withdraw it later.
As per Assange I have since changed my opinion about him: he clearly affirmed that by publishing the e-mails on Wikileaks he wanted to help Trump be elected. Not that he was a fan of him, but he was more opposed to Hillary Clinton. I am not a fan of Hillary either, but I don't believe the exaggerated conspiracies around her and her husband. And now Assange is reaping what he sow. Sorry for him, but he is a danger for the country not a help, by his irresponsible behaviour.
The effect of Russian meddling in the campaign has been studied in detail by an expert and she has concluded that it is highly like that that interference changed the course of voting in decisive favour to Trump:
"Few people, including Trump’s opponents, have publicly challenged the widespread belief that no obtainable evidence can prove that Russian interference changed any votes.
The U.S. intelligence community, for its part, is prohibited from investigating domestic political affairs. James Clapper, the former director of National Intelligence, speaking for himself, however, told me that “it stretches credulity to think the Russians didn’t turn the election.”
Politicians may be too timid to explore the subject, but a new book from, of all places, Oxford University Press promises to be incendiary. “Cyberwar: How Russian Hackers and Trolls Helped Elect a President—What We Don’t, Can’t, and Do Know,” by Kathleen Hall Jamieson, a professor of communications at the University of Pennsylvania, dares to ask—and even attempts to answer—whether Russian meddling had a decisive impact in 2016. Jamieson offers a forensic analysis of the available evidence and concludes that Russia very likely delivered Trump’s victory.
As for the evidence about Russian meddling in the election here you a have a full article with all details: