"The evidence establishes, therefore, that even when Republicans are an electoral minority, their legislative power remains secure," Ripple wrote.

Republicans have a modest natural advantage geographically, because Democrats tend to be concentrated in Milwaukee and Madison, Ripple wrote, but that doesn't explain the partisan effect of the 2011 redistricting.

GOP drafters created and passed on several plans that would have been more modest, Ripple wrote, concluding that the burden on Democratic voters imposed by the 2011 redistricting plan is not justifiable.

In his dissent, Griesbach wrote that he could not accept proof of intent to act for political purposes as a part of any test to determine whether a redistricting plan was unconstitutional.

"If political motivation is improper," he wrote, "then the task of redistricting should be constitutionally assigned to some other body, a change in law we lack the authority to effect."

He said the Democrats' case was also "a poor vehicle for the remedying of any grave injustice" because it's likely that Republicans would have won control of the Legislature in 2012 and 2014 "even without the alleged gerrymandering."

Griesbach also said that the efficiency gap is inadequate as a measure of partisan gerrymandering, and that it fails to adequately account for the political geography in Wisconsin, in which Democrats are naturally packed into urban areas like Madison and Milwaukee. He also called the efficiency gap "highly volatile" and possibly triggering court interventions in redistricting when it isn't necessary.

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