By Steve Barnes
LITTLE ROCK, Ark., Sept 8 (Reuters) - The Arkansas race for a seat in the U.S. Senate is nearly a dead heat and almost certain to be the most expensive in the state's history as Democrats and Republicans pour money into a battle that could help determine the balance of power in the body.
Apart from the spending, a tipping point in the campaign could be whether distaste for President Barack Obama outweighs reverence for the Pryor family, a state political power for decades, analysts say.
The contest pits two-term Democrat Mark Pryor, an ally of the state's favorite political son, Bill Clinton, against Rep. Tom Cotton, a Republican who returned to Arkansas only two years ago to win a seat in the House of Representatives.
"It's a knife fight to the end," said Don Tilton, a veteran Little Rock lobbyist. A poll last week showed Cotton slightly ahead, but with a lead smaller than the margin of error for the survey.
The campaigns and interest groups already have spent almost $12 million on television advertising, a state record for a U.S. Senate race.
Former President Clinton, who served as Arkansas governor, has made several appearances in support of Pryor. Mitt Romney, the 2012 Republican presidential nominee, has campaigned for Cotton.
Most polls have reflected a disdain among voters for Obama, which observers say is the core of Pryor's difficulty.
"Obama is still poison here," Tilton said.
Pryor is a third-generation Arkansas politician whose father, David Pryor, served as a congressman, governor and U.S. senator.
After keeping a low profile during previous Senate races, the elder Pryor has taken a visible role this year, making speeches and appearing in a TV commercial with his son.
Harvard-educated Cotton has been trying to tie Pryor to a different politician, President Obama.
Cotton has attacked Pryor for his support of one of the Obama administration's center-piece policies, the Affordable Care Act, also known as "Obamacare."
Arkansas drew up its own, bipartisan version of Obamacare, with 200,000 previously uninsured citizens enrolling in the state's plan, which uses federal funds to subsidize health coverage purchased from private carriers.
"Pryor has been a centrist, which has always been the key to political success in Arkansas," said Angie Maxwell, a professor of political science at the University of Arkansas.
"It will be interesting to see if the center has shifted that far to the right."
(Writing by Jon Herskovitz; Editing by Dan Grebler)