From the Green Party of Colorado, via Joshua Fauver at American Third Party Report:
This morning, the Green Party of Colorado received word that the Green Party of Oklahoma was endorsing Bernie Sanders for president.
Almost immediately, we here at GPCO began fielding messages and calls, encouraging us to do the same. Therefore, we need to clearly state that we will not be endorsing Sanders, for both electoral and political reasons, which we will outline here.
First, voters need to understand that Oklahoma is an open-primary state. Colorado is not. What does that mean? Very simply, Oklahoma’s election laws allow independents to vote in any party’s primary. Currently, there are only two recognized parties in Oklahoma: the Republicans and the Democrats.
Conversely, in Colorado, independents may not vote in partisan primaries. The Green Party of Colorado is one of the many recognized political parties in this state, and we have permanent ballot status. This means that our candidates do not have to get petition signatures to appear on the ballot, whereas independent candidates do.
Additionally, we are affiliated with the Green Party of the United states. One of the requirements of accreditation with GPUS is that we must support only the presidential nominee from our national party. We do not intend to do anything but that. We have five different presidential candidates, and from among them, we will choose our nominee at our national convention in Houston on August 3-7, 2016. Colorado will send five delegates to that convention. At present, Dr. Jill Stein is recognized as the front runner, but the other four candidates are also very thoughtful and forward thinking, much more than anything the Democratic slate has to offer.

The requirement that green party members can only support the green party nominee is the reason why my wife and I are changing our registration from green party to independent. It’s about being able to follow progressive ideology that comes from any candidate no matter their party affiliation. Lookup dogma and you’ll find the definition of inflexibility.