During the election in which Barack Obama first ran for president, he ran against Hilary Clinton in the democratic race. After resigning from the election, Hilary Clinton and her supporters supported the election of Obama vs McCain. After his term as president, George Bush supported McCain during the election in hopes of getting Republicans re-elected in the House.
Yes, open primaries would be consistent with the ideals of free association. I believe that open primaries would allow for more freedom of choice within elections. It would allow those who might want to nominate a candidate for a different party instead of their own. This would also allow for more than just two main party candidates to be elected to office.