The Denver Post reported that the suit would be filed Monday.
Among those challenging the Taxpayers Bill of Rights - or TABOR - are Democratic Rep. Andy Kerr of Lakewood and former Republican state Sen. Norma Anderson.
TABOR requires lawmakers to ask voters to raise taxes. It also limits the amount of tax revenue the state can keep and spend.
The suit alleges that it violates the U.S. Constitution's guarantee of a small-R "republican" government where elected representatives make decisions on behalf of voters.
The plaintiffs are being represented by Boulder attorney Herb Fenster, a Republican, along with two Democrats, former Congressman David Skaggs and former state Sen. Mike Feeley of Lakewood.
(Copyright 2011 by The Associated Press. All Rights Reserved.)