Everybody has been assuming that Wisconsin Republicans can and will have Democratic State Senators taken by the police if they return to the state. Fourth Branch has a great legal rundown showing that it would be illegal to do so:

Unfortunately for Senate Republicans, there is no law or rule which would permit the Wisconsin State Police to compel a Democratic Senator to return to the Capitol Building. Actually, the analysis is even more concrete: the Wisconsin Constitution prohibits the arrest of any member of the Wisconsin Senate while the legislature is in session. As provided in Article 12, Section 15:
Members of the legislature shall in all cases, except treason, felony and breach of the peace, be privileged from arrest; nor shall they be subject to any civil process, during the session of the legislature, nor for fifteen days next before the commencement and after the termination of each session.”
The Democratic Senators are not committing treason, a felony or breach of the peace by avoiding the Senate. There may be political ramifications (including a recall election, for example), but that’s a far cry from asserting criminal ramifications.
Without the power to compel Democratic Senators to return to the Capitol, the State Police could find themselves in violation of Wisconsin criminal laws- possibly including kidnapping.

There's more to it, and the whole rundown is worth a read.