The closer is example is 2004, and it's really not that close. The president of Diebold -- that year's voting machine contractor -- supposedly promised he would deliver Ohio for G. W. Bush. Election Day exit polls, which have generally been the most accurate, suggested Kerry was rapidly closing on Bush, but the final tallies did not reflect that. So a couple conspiracy theories bounced around, but they got little to no traction among Democrats and the media. Bush won. Kerry lost. We moved on.
Of course the 2000 election was razor thin, with a Republican secretary of state calling the shots, and a conservative Supreme Court majority not allowing for the kind of recount Republicans now deem perfectly reasonable and necessary, in an election that was declared early for the candidate with fewer votes. Republicans bussed hired protestors to Florida to pretend outrage, but Gore and the Democrats accepted the results and moved on.
You know who didn't accept the 2016 results? Donald Trump. He was so sure he was going to lose that he pre-declared the election fraudulent. Until he won. Then he realized Hillary Clinton had 3 million more popular votes. Then he declared it fraudulent again, claiming proof that illegal immigrants had voted for Hillary, pulling the number 3 million out of his a$$. All through this, I don't recall Democrats claiming Trump had secured the Presidency illegally, even if we couldn't believe it had happened and hated what it meant about the country. I just remember thinking that a lot of Americans had kept quiet about their support of Trump, and used the anonymity of the voting booth to vex the pollsters. Most Democrats spent zero time on conspiracy theories, and more time wondering if Hillary Clinton wasn't the wisest choice.
There's no doubt Russia tried to influence the U.S. election for Trump. There's no damning evidence of collusion from the Trump team. There's less evidence it actually swung the election. Russia mattered mostly because the intelligence agencies that monitored Trump knew the unsavory company he kept in Russia and Eastern Europe, and how his compromised finances presented a security risk if he became President of the United States.