DANVILLE, Va. -- The challenge facing Rep. Tom Perriello can be seen all along U.S. 58 where Republican campaign signs outnumber those for Democrats 7-to-1.

Mr. Perriello is a freshman Democrat.

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Freshman Democratic Rep. Tom Perriello at an August town-hall meeting on health care at Fluvanna Middle School in Fork Union, Va.

As both parties gear up for next year's congressional elections, the most vulnerable candidates are those among the 49 primarily freshmen and sophomore Democratic House members from districts that supported the Republican presidential nominee, Sen. John McCain, in the 2008 election. Republicans rank Mr. Perriello, who squeaked out a victory in this southern Virginia district by 727 votes, as perhaps their...