Republicans need a net gain nationally of four seats to win Senate control, and Democratic Sen. Ben Nelson's decision to retire in Nebraska left the GOP nearly certain of a pickup until Kerrey entered the race. However, Republicans remain confident they can beat the former Nebraska senator, governor and 1992 presidential candidate who served as a Navy SEAL in Vietnam and won the Medal of Honor.
In the presidential race, likely GOP nominee Romney spent his day in Iowa, a competitive general election battleground, criticizing Obama on voters' top concern, the economy.
"This is not solely a Democrat or a Republican problem," Romney said in Des Moines in a clear pitch to independent voters who will decide the election. "The issue isn't who deserves the most blame, it's who is going to do what it takes to put out the fire."
The White House promptly dismissed Romney's critique.
Press secretary Jay Carney blamed federal overspending primarily on Romney-backed tax cuts for the wealthy that were enacted during President George W. Bush's administration and on the pricey wars in Iraq and Afghanistan.
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