I honestly don’t know whether the so-called collusion in this case was a crime, in part, because this story keeps morphing. Trying to get dirt on an opposing candidate is not necessarily a crime. However, making false statements to government officials can be. I think the more likely crimes have occurred through false and misleading statements to government officials throughout this probe, but Robert Mueller will have to determine whether that occurred. Generally, it is the easiest crime to prove.
Finally, it is good to keep in mind that there is no crime of “collusion” in the federal code. The applicable crime is conspiracy under 18 USC Sec. 371. That would cover a conspiracy by two or more persons to violate a law of the United States or “to defraud the United States.” You need an election law specialist to tell you whether asking the Russians for negative information on Hillary Clinton violated federal law. The question of whether there has been a scheme “to defraud the United States” is a more interesting one. Generally, that portion of the statute is used for financial crimes. However, who knows what a creative prosecutor might seek to use it for?