I guess if you don’t count Hillary Clinton saying he was an illegitimate president

https://bangordailynews.com/2019/09/26/national-politics/hillary-clinton-trump-is-an-illegitimate-president/

https://www.independent.co.uk/news/world/americas/us-politics/hillary-clinton-trump-impeachment-illegitimate-president-2016-election-cbs-a9122986.html?amp

And then there was the whole ridiculous Russia conspiracy theory pushed by Democrats.

https://nypost.com/2020/11/10/roots-of-stolen-election-claims-and-other-commentary/amp/

Then there was Jimmy Carter

https://www.vox.com/platform/amp/2019/6/28/19127475/jimmy-carter-trump-illegitimate-president

Here is more

https://amp.theguardian.com/commentisfree/2017/nov/08/donald-trump-illegitimate-president-rebecca-solnit