A poll released last week showed Sen. Dick Lugar 10 percentage points behind challenger Richard Mourdock, Indiana's state treasurer.
tea party and other groups have proclaimed Lugar to be too moderate and too willing to work with Democrats.
The race has drawn the attention of high-profile political figures, with Mourdock getting endorsements from former GOP presidential candidate Rick Santorum, U.S. Rep. Michele Bachmann of Minnesota and former Alaska Gov. Sarah Palin.
Lugar has been backed by the U.S. Chamber of Commerce, U.S. Sen. John McCain of Arizona, Indiana Gov. Mitch Daniels and pop singer Pat Boone.