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The Pryor campaign announced Monday that it had raised $1.2 million in the second quarter of the year, and now has $4 million for his re-election fight. Mr. Cotton, meanwhile, raised $611,000 and has more than $1 million cash on hand.

“If and when Cotton gets in, I expect this thing to be a battle royale,” Mr. Reed said.

Republicans, meanwhile, are lining up to challenge Sen. Mark Begich in Alaska.

Alaska Lt. Gov. Mead Treadwell and tea party favorite Joe Miller have announced their candidacies and former Alaska Gov. Sarah Palin told Fox News she has “considered” a run.

In Louisiana, Rep. Bill Cassidy and Rob Maness, a retired Air Force colonel, are seeking the Republican nomination to run against Democratic Sen. Mary L. Landrieu, who is running for her fourth term.

And in North Carolina, state House Speaker Thom Tillis and Greg Bannon, a tea party-backed candidate, are running for the GOP nod to challenge Sen. Kay R. Hagan, who rode into office in 2008 on the coattails of Mr. Obama’s historic victory.