A question of legitimacy
For about six years, Donald Trump led a bogus challenge to the legitimacy of the presidency of Barack Obama, questioning where Obama was born. Democrats rightly criticized Trumpís birtherism, not only because the assertion of Obama being born in Kenya is absurd, but also because challenging the legitimacy of the democratic process potentially damages that process.
Trumpís irresponsible use of language doesnít justify Democrats using that same language. The question of legitimacy of the Donald Trump presidency arises from the fact that the Russians attempted to influence the outcome of the election and potentially succeeded.
In any national election, there are many factors which have an impact on the election, including unethical acts by supporters of a candidate. As far as can be determined, Trump was not in collusion with Russians to affect the election.
If we want to go down that road, there is probably a stronger case to be made that the Democratic nomination of Hillary Clinton was illegitimate because the Democratic National Committee fed debate questions to Clinton prior to her debates with Bernie Sanders.
Democrats will have many substantive issues on which to challenge Trump. The Democrats will be more effective by avoiding involvement in semantic mud fights.