Dueling partisan polling shows leads for both candidates while external polling has shown Schauer closing the distance in what has generally been considered a strong pick-up opportunity for Democrats looking to increase their majority in the U.S. Congress.
Schwarz told The Associated Press on Tuesday that he decided to endorse Schauer over Republican Rep. Tim Walberg because the anti-tax Club for Growth began running ads critical of Schauer.
The Club for Growth injected more than $1 million into the 2006 primary, helping Walberg defeat Schwarz, then a first-term congressman.
Schwarz says he couldn't remain neutral in the race once the group got involved. He says it's "the straw that broke the camel's back."
A debate between Schauer and Walberg is set for October 7.