By Matthew Chapman On Monday, the Associated Press profiled independent voters in Wisconsin who generally lean conservative — but have soured on Republican Sen. Ron Johnson. Johnson is locked in a tight race with Democratic Lt. Gov. Mandela Barnes in one of the most closely-watched Senate contests of the election. A recent poll by Marquette University found Johnson with a razor-thin lead after trailing previously, and an improvement with independent voters driving much of that movement. Nevertheless, the report noted, many of these voters are up for grabs — and Johnson is still struggling with…