Hell yeah, it is.
The First Amendment of The U.S. Constitution protects "Freedom of association." Taking away the ability of the Wisconsin Senate Democrats not to assemble with State Republicans is an act against the U.S. Constitution.
There's nothing in Federal law, which always trumps state law, that says a legislator must be present for every vote on every bill. Heck, look at how the conservative Erick Erickson was slammed for abstaining all of the city council meetings he missed while being on the Macon City Council. Under the logic of his blog's attack on the Wisconsin Senate Democrats, Erickson himself should have been arrested.
And they should be locked up for their misquoting Wisconsin Senate Rule 23. They have printed, again and again, this:
Senate Rule 23
Senate Rule 23. Committee not to be absent. Members of a committee, except a conference committee, may not be absent by reason of their appointment during the sitting of the senate, without special leave.
[am. 2001 S.Res. 2]
Which, if you look at the Red State blog post by SusanAnne Hiller, has a link that takes you to a web page indicating that the rule has expired. Now, laugh for a minute, then continue reading below for more laughter.
If you take that entire "rule" and copy it and paste it as a search in the Wisconsin Senate Data Base, you can't find it.
The bottom line is Wisconsin has no authority to arrest Senate Democrats. None. Moreover, it seems the Wisconsin Government has become a police state, with cops tackling Representative Nick Milroy for just coming in to get his clothes:
Wisconsin should vote to recall Governor Scott Walker. This is nuts. Governor Walker should give up his obviously personal attack on teachers.