While attack TV and radio ads from U.S. Rep. Peter DeFazio and Republican challenger Art Robinson are being hurled daily at Douglas County voters, a more subtle battle is taking place behind the scenes. Republicans and Democrats are calling voters to encourage them to fill out and return their ballots. Both parties have spent the past few weeks poring over voter registration lists and targeting party members to ensure they vote. “We’re concentrating on voters who were registered and received a ballot, but did not vote in the primary,” said Linda Hellenthal, chairwoman of the Douglas County Republican Party. Hellenthal …