The following is a quote from my textbook used in the Forensic Linguistics class I took last semester (Solan, L. M. & Tiersma, P. M. (2005). Speaking of Crime: the Language of Criminal Justice. Chicago University Press, p.181.):
Not only is it illegal to commit a crime, but people can also be punished for asking or inducing someone else to do so. This is the crime of solicitation...The state must usually prove that the solicitor intended the crime to be committed, although the crime does not actually have to be carried out. What is essential, at least under federal law, is that the solicitor "solicits, commands, induces, or otherwise endeavors to persuade"(1) someone else to engage in the crime. The essence of solicitation is language.
The speech act that the defendant must have performed is a request, or perhaps an offer or command.
1. 18 U.S.C. § 373 (2000)
Whereas his likely tax fraud and his near-certain collusion with Russia remain hidden due to his relentless obstruction of justice (a prosecutable offense in itself), his crime of solicitation is now a matter of public record (WSJ, BBC, Fox News). Using one's office to launch a very dubious investigation into one's political opponent in an upcoming election is downright corruption, not to mention a huge conflict of interest.
His continued bullying of witnesses on Capitol Hill is also a matter of public record (BBC, Reuters), as well as his ambiguous allusion to "handling spies" such as the whistleblower. This is almost certainly witness tampering. I now present another quote from Solan & Tiersma (p. 204):
Indirect and Ambiguous Threats
Threats - like other speech acts in general and like crimes of language in particular - tend to be made indirectly. Alternatively, they may be phrased in ambiguous terms to give them plausible deniability. As with other speech acts, pragmatic factors count for a lot in determining whether an utterance is a threat or something else.
Regardless of your politics, the highest office in the land is, and rightfully ought to be, accountable to the law. Whether the Senate Republicans abide by their constitutional oath or not remains to be seen, but even if they don't, the man is absolutely guilty.
There is no rational defense.