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Labor union supporters for Mrs. Carnahan sponsored a radio ad saying Mr. Blunt has repeatedly supported pay raises for himself and other members of Congress. Lawyers forced a Springfield radio station to postpone the spot until it passes a legal review. Environmental groups have poured money into the race on Mrs. Carnahan’s behalf.

Mr. Blunt has played up his opposition to Mr. Obama’s health care overhaul and the $814 billion economic stimulus program - at a time when Mr. Obama’s approval rating in Missouri is just 34 percent, roughly 9 percentage points below the national average.

Mr. Obama narrowly lost the state to his Republican competitor, Sen. John McCain of Arizona, in the 2008 presidential election, but has had a tough time maintaining support. In Missouri’s first-in-the-nation voter referendum earlier this month, more than 71 percent of voters opposed the insurance-buying mandate in Mr. Obama’s health care plan.

“We are going to continue talking about the issues of the campaign - that Robin Carnahan is a rubber stamp for the failed economic policies of [Senate Majority Leader] Harry Reid, [House Speaker] Nancy Pelosi and Barack Obama,” said Blunt campaign manager Rich Chrismer.

The Carnahan campaign did not respond to offers to comment for this article.

She has been willing to break with Mr. Obama and her party. She criticizes the Troubled Asset Relief Program that bailed out Wall Street financial institutions despite Mr. Obama’s vote in favor in 2008 in the Senate, and she recently broke with the president to support an extension of George W. Bush-era tax cuts, even for the highest income brackets.

Even though Democrats control the White House and both houses of Congress, a recent Carnahan attack derided the Republican candidate as the “very worst of Washington.”

If the 2010 primaries are an indication, Mr. Blunt should win the independent vote, though getting an accurate number is difficult because residents do not register by political party and exit-poll estimates have varied wildly in recent years.

University of Missouri professor Mark Overby said independents represent 10 percent to 20 percent of voters in the state, while Mr. Coker estimates that it could run as high as 30 percent.

Mr. Blunt should win in the conservative southwest region where he lives, and Mrs. Carnahan is likely to get more votes in the more liberal-leaning cities of St. Louis and Kansas City. State political analysts say the race could be decided in the voter-rich suburbs of St. Louis and that voter turnout in that region will be key.

Mr. Overby acknowledged the criticism that Mrs. Carnahan has been slow to hit the road and meet voters, but added that it might be good strategy.

Mrs. Carnahan hasn’t fully unlumbered her campaign, but should start around Labor Day,” Mr. Overby said. “Maybe’s she smarter than we think.”