Not illegal, but a bad act if you’re going to run for President. Very bad if you let it influence your decisions. Trump signed a letter of intent on Oct 28, 2015. He would spend his candidacy, transition, and 23 months in office denying it. During the campaign, Trump lied to his voters and said he had nothing to do with Russia. He said no one at Trump Org, or with the campaign has any business with Russia. That was a lie.
Verified: No longer in dispute. The document is now public. CNN’s Chris Cuomo uncovered the document and published it on Dec 19, 2018.