When news of the case first surfaced, constitutional law types (myself included) thought it was frivolous. The United States in a 1912 case involving Pacific Gas and Electric held that a challenge of Oregon's initiative process as violating the Guarantee Clause was a non-justiciable political question. The challenge here of Colorado's TABOR amendment, adopted by the Citizens of Colorado through the initiative process, certainly seemed to be equally non-justiciable.
After the case was filed, Colorado Attorney General John Suthers moved to dismiss it, as he should. In what can only be described as perhaps the judicial shocker of the decade, Judge Martinez, the newest Colorado federal district court judge appointed by President Obama, in a 73-page opinion, refused to dismiss the Guarantee Clause claim and further turned back a challenge to the standing of certain individual legislators to bring the case. On this latter point, it should be noted that the General Assembly itself is not bringing this case, nor did it vote to authorize the particular legislators to bring the case.
Just exactly what is a Republican Form of Government that the federal court held could at least theoretically be violated by TABOR? We have a General Assembly, consisting of a state House of Representatives and a State Senate. They enact all sorts of laws and exercise the State's police power that is the core of state sovereignty. They enact legislation appropriating approximately $20 billion in state and federal funds annually. Is this not "Republican in form"?
There is no measuring stick except for the fact that, per Judge Martinez, the legislators have alleged that they should be allowed to raise taxes without first seeking voter approval and the denial of such power is denial of a "core" legislative power that they should enjoy.
The Attorney General just filed a motion asking the court to certify the case for an interlocutory appeal. In other words, the AG is asking the district court to allow the Tenth Circuit to weigh in before this goes off on the frolic and detour that will ensue if the case is allowed to proceed to trial. Let us hope that the State and Colorado's citizens who support having a voice in the raising of their taxes get the second look they deserve.