The Constitutional Union Party (also known as the Bell-Everett Party in California) was a political party in the United States created in 1860. It was made up of conservative former Whigs who wanted to avoid disunion over the slavery issue. These former Whigs (some whom had been under the banner of the Opposition Party in 1854-58) teamed up with former Know-Nothings and a few Southern Democrats who were against disunion to form the Constitutional Union Party. Its name comes from its extremely simple platform, a simple resolution "to recognize no political principle other than the Constitution...the Union...and the Enforcement of the Laws." (wikipedia)
That principal, simply put, was to enforce the laws of the land as they were written. While it was not a vote for secession, it was a vote for Constitutional governance by the rule of law.