The Denver Post is now reporting that the state GOP will not be suing to keep Tom Tancredo off of the American Constitution Party ticket despite earlier assertions that they would.
Besides the PR disaster of the GOP suing Tancredo and thereby feeding oxygen to the story and Tancredo the legal merits of a claim would be pretty suspect.
The parties are free to make any eligibility requirements that they like, so long as there is no conflict with state statute and they follow those eligibility requirements. The GOP General Counsel, and law school classmate of mine, Ryan Call, admitted that there is no statute that speaks to this issue.
I'm also not sure what standing the state GOP would have to sue another political party in such a situation. Presumably they would have to find a GOP friendly American Constitution Party member to bring the suit and then we're back to the same issue as above - on what grounds?