There is actually well documented proof that he is a Russian asset.

https://www.businessinsider.com/russia-cultivating-trump-asset-40-years-says-ex-kgb-spy-2021-1

https://nymag.com/intelligencer/article/ex-kgb-agent-trump-russian-asset-mueller-putin-kompromat-unger-book.html

https://en.wikipedia.org/wiki/Links_between_Trump_associates_and_Russian_officials

https://www.timesofisrael.com/kgb-groomed-trump-as-an-asset-for-40-years-former-spy-says/

And finally, the argument that he is more a useful idiot than an "Agent", because agents are normally clever:

https://theconversation.com/donald-trump-spying-allegations-more-likely-useful-idiot-than-putins-agent-154300