Russian hacking to interfere in the presidential election deserves investigation and possibly punishment. But even if it swayed the race for Trump, it doesn’t make his election illegitimate. The hacking gave voters extra information, albeit wrongly obtained information, and they were able to weigh that information and its likely origin–which was widely discussed during the campaign–in making their decisions. An electoral majority of them decided for Trump. When it comes to the legitimacy of the election outcome, what ought to be decisive is that the voters made uncoerced choices that were tabulated correctly and translated into electoral votes by the long-accepted rules.*
The Democratic fixation on the idea that some combination of Russia and the FBI unfairly swayed the election runs the risk of distracting Democrats from grappling with the fact that Hillary Clinton created a situation where it could be swayed. Some of her mistakes are water under the bridge: Presumably next time around Democrats will not nominate someone under FBI investigation. But other mistakes, like avoiding Wisconsin and not trying to win over a few more white evangelical voters, could be repeated. The truth of the matter may be that both Russian intervention and Clinton’s terribleness were necessary conditions for Trump’s victory. To the extent Democrats settle on the story that Clinton is the victim of a stolen election, they may be less likely to adapt to avoid running terrible campaigns in the future.
* There is a very strong case that the electors, on the original understanding of the Constitution, were supposed to have the discretion to reject potential presidents they considered unsuitable, and a strong case that state laws that attempt to bind the electors are unconstitutional. Accepting both cases is compatible with thinking that for the electors to exercise their discretion to undo the electoral majority’s decision would be catastrophically unwise.