Anyone who is qualified can run-you or I could declare as candidates for a party. The Party has no say in it, at that point, and cannot control who throws their hat into the ring. How The Party (Dem, Repub or other) reacts and supports a declared candidate further in the process is a different issue than the current situation with Tancredo, who may or may not be successful as a candidate.  If he is we will have to wait and see just how much “The Party” gets behind him and influences the outcome later in the process. Bernie Sanders can probably expound on that at length.