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As it stands, the battle for a spot in a runoff race has boiled down to a three-person contest of Rep. Jack Kingston, deep-pocketed businessman David Perdue and former Secretary of State Karen Handel.

That could be a bad omen for Democrats, as polls show likely Democratic opponent Michelle Nunn was performing better against Reps. Phil Gingrey or Paul C. Broun, who were seen as the most conservative candidates in the race.

But Mrs. Kingston, Mr. Perdue and Mrs. Handel have engaged in an increasingly vicious campaign, which is likely to continue for another 90 days until the July 22 runoff.

“The question will be: Will the next nine weeks be a civil period or one of scorched earth?” Mr. O'Connell said. “If it is scorched earth, then too many open wounds on the eventually GOP victory could give Nunn an opening.”

In other key races across the country Tuesday, freshman Rep. Tom Cotton, a veteran of the wars in Iraq and Afghanistan, is expected to win the Republican Senate nomination in Arkansas, putting him on a collision course with two-term Sen. Mark L. Pryor — one of the most vulnerable Democrats in the midterm elections.

In Idaho’s 2nd Congressional District, Rep. Michael K. Simpson is positioned to defeat Bryan Smith, a political novice aligned with the tea party.

In Pennsylvania, Rep. Allyson Y. Schwartz, state Treasurer Rob McCord and free-spending businessman Tom Wolf are running to face off against Gov. Tom Corbett, one of Democrats’ top gubernatorial targets.