About 1.6 million people in North Carolina, or one-fourth of the electorate, are registered to vote but not affiliated with a party.
Paul Shumaker, a Republican consultant who has handled statewide races, said those voters may present just as many problems for Romney as the Democrats.
"Having to go through a Republican primary that pulled him to the right hurts him in building a winning coalition in the state," Shumaker said.
Theresa Jeffers is one of the unaffiliated voters siding with Romney — though she says it's more a vote against Obama than in favor of the Republican.
"I was hoping so much that he would be successful," Jeffers, 64, said of the president. She said she's been disappointed by his performance in office, particularly on the debt.
Romney is hoping millions of dollars in television advertisements will convince more voters like Jeffers to side with him. The candidate and his GOP allies have spent more than $32 million on ads in North Carolina, including $2 million last week alone.
Obama and Democrats have spent nearly $18 million on North Carolina advertising. But their rate of weekly spending has slowed, down about half from their peak in the summer. Obama's campaign is also not running the costly two-minute ad it released Thursday in the state.
Neither candidate has devoted much time on the ground in North Carolina. Romney has made four trips to the state, while Obama has been here three times, including during the Democratic Party's convention in Charlotte earlier this month. But their running mates and wives have been frequent visitors.
Obama had planned to deliver his acceptance speech at a Charlotte football stadium with seating for 74,000 and use the gathering for voter registration and recruitment. But the outdoor event was canceled, with Democrats blaming the weather and Republicans accusing the president of not being able to draw a capacity crowd.
Associated Press writer Charles Babington in Washington contributed to this report.
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