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In Michigan, a poll by a Democrat-leaning firm showed Mr. Romney leading by 4 percentage points last week, 48 percent to 44 percent. But some analysts said they want to see more data from Michigan; a different poll taken just a few days earlier showed Mr. Obama with a 5-point lead there.

In some key Senate races, Republicans are improving their positions, according to polls. In Wisconsin, former Gov. Tommy G. Thompson leads in two surveys over his Democratic opponent, Rep. Tammy Baldwin — by 9 points in one poll, 5 in another.

In Connecticut, Republican Linda McMahon has taken the lead in the Senate race against her Democratic rival, Rep. Christopher S. Murphy. A Rasmussen poll showed her with 49 percent support, compared with 46 percent for Mr. Murphy.