A federal appeals court on Friday said it would allow a lawsuit challenging the constitutionality of Colorado’s Taxpayer’s Bill of Rights to go forward.

The decision by the 10th U.S. Circuit Court of Appeals marks a milestone in a legal fight over how Colorado conducts its most important functions.

For more than two years, Attorney General John Suthers has argued that plaintiffs in the lawsuit did not have the right to sue. The lawsuit, filed by more than two dozen individuals, argues that by taking away lawmakers’ ability to tax, TABOR violates the U.S. Constitution’s guarantee that every state have a republican form of government.

Colorado voters in 1992 passed the TABOR amendment, which limits state spending and bars lawmakers from raising taxes without a vote of the people.

The suit was brought against Gov. John Hickenlooper in May 2011.

In 2012, Suthers was rebuffed in his argument by a U.S. District Court judge, who ruled the plaintiffs had a right to sue.

The plaintiffs, who include some current state lawmakers such as Sen. Andy Kerr, D-Lakewood, and House Majority Leader Dickey Lee Hullinghorst, D-Boulder, claims that TABOR deprives them of their ability to perform the “legislative core functions of taxation and appropriation.” Moreover, the plaintiffs say explicitly that TABOR prevents them from doing their jobs.

In Friday’s 49-page decision, the 10th U.S. Circuit notes the “Supreme Court has held that members of a state legislature may have standing to sue in order to vindicate the ‘plain, direct and adequate interest in maintaining the effectiveness of their votes.’ “

The 10th Circuit’s decision on Friday did not rule on the merits of the case.

“This now opens the path for us to go back to the trial court to test whether Colorado retains a republican form of government,” said Mike Feeley, a lawyer for the plaintiffs.

Suthers could appeal the decision to the U.S. Supreme Court.

A spokeswoman for the attorney general said Friday the office is reviewing the ruling and has not made a decision on what will be the next step.

Kurtis Lee: 303-954-1655, klee@denverpost.com or twitter.com/kurtisalee

The taxpayer’s bill of rights

An amendment to the Colorado Constitution that places limits on government revenue growth and requires that excess revenue be returned to taxpayers unless voters agree to let the government keep the excess.