Republican efforts to take back the Senate – already complicated by the controversial campaign of Todd Akin in Missouri and a tight race in Massachusetts – could face another challenge in one of the most unexpected of places.

In Arizona, Democratic candidate Richard Carmona is polling surprisingly well against his Republican challenger, Rep. Jeff Flake. The RealClearPolitics average of polls puts Congressman Flake up by 2.3 percentage points, 44.3 to 42 percent.

On Wednesday, Mr. Carmona's campaign released a poll that suggested Carmona was actually ahead, 47 to 43 percent. Within hours, the Flake campaign countered with its own poll, suggesting Flake was up 49 to 43 percent.

"All you can say at this moment is that it's a race worth watching," says Larry Sabato, a professor of politics at the University of Virginia.

It remains an uphill battle for the Democrats. Arizona hasn't sent a Democrat to the Senate in nearly 18 years and hasn't voted for a Democratic candidate for president since 1996, when it went for Bill Clinton. Wednesday evening, Mr. Clinton will stump for Carmona at a rally in Tempe, Ariz.

Carmona and Flake, a sixth-term congressman known as a fiscal conservative, are vying for the open seat of retiring Republican Sen. Jon Kyl.

Carmona's rise in the polls has come even as Mitt Romney has solidified his lead over President Obama here. A virtual unknown outside of southern Arizona, he has become competitive mostly through a compelling personal narrative. He grew up in Harlem, dropped out of high school, and joined the Army at 17. He later became a decorated Vietnam War veteran, a surgeon general under George W. Bush, a SWAT team member, and a medical doctor.

"His ads have focused on his résumé, which is pretty exceptional," says Bruce Merrill, a political scientist and public-opinion researcher in the Phoenix area. "He worked his way up from nothing."

Carmona also is of Puerto Rican descent, something the state Democratic leadership is banking on to attract Hispanic voters. Their national counterparts have devoted more than $1 million to the race, forcing Republicans to spend roughly half that amount to boost Flake's campaign. They have portrayed Carmona as "Barack Obama's rubberstamp."

It could be an effective strategy. "His association with Obama in a state that isn't that favorably exposed to him is not a good thing," says Margaret Kenski, an Arizona pollster in Tucson who works mostly with Republicans.

Obama is not popular among many Arizonans who "don't like Obamacare and feel that he hasn't done enough on border issues," she says.

But Flake's highly divisive primary contest came at a cost and seems to have opened a window for Carmona, who ran unopposed, Ms. Kenski adds.

To win, Carmona would have to win over a majority of independent voters and get a significant number of Hispanic voters to the polls, says Mr. Merrill, the political scientist. About a third of Arizona voters are registered as independent, while about 36 percent of Arizona's voters are Republican, and 30 percent are Democrat. Hispanics make up 30 percent of Arizona's population.

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Carmona has targeted independents with his ads. "Republicans and Democrats both got it wrong," he says in one spot. "We've got to make health care affordable to small business and working families through innovation and preventive care while also cutting waste and fraud."

But Kenski notes that Carmona has yet to make clear his stance on health care and other issues of concern to Arizonans, which could hurt his chances.