What are the Senate (and the House, for matter) rules for a voting quorum? The reason I ask is, Democrats in both Indiana and Wisconsin—within the past year—have walked out of legislatures to prevent the business of government from moving forward. Should that action too, be considered un-Constitutional?
It depends on the state constitution. They govern and make their own rules. That's why several states have balanced budget ammendments and others run deficit spending as the everyday norm. It's aslo why WI was able to get around it and why IN could not. It depends on each state's own constitution.
“The reason I ask is, Democrats in both Indiana and Wisconsin—within the past year—have walked out of legislatures to prevent the business of government from moving forward. Should that action too, be considered un-Constitutional?”
That’s a state issue, and has nothing to do with the Constitution. Unless you want to argue them walking out deprived the people of a republican form of government. But that part of the Constitution, so far as I know, has never been enforced.