By Andrea HopkinsMONROE CITY, Indiana (Reuters) - In a dozen districts across the U.S. heartland, voter unhappiness has imperiled Republicans, setting the stage for what could be the biggest anti-incumbent midterm election since 1994.Pat Wilkerson says U.S. troops and veterans are her first priority, believes family values are important and voted Republican in 2004. But in November she'll switch parties -- though not because Democrats have won her over."When I vote now, it's not who I'm voting for, it's who I'm voting against," said the 59-year-old administrator, adding she is fed up with the war in Iraq and wants troops...