Senate Democrats' campaign arm on Tuesday began a $3.6 million ad blitz against Republican Tom Cotton as one of the longest and most intense Senate races in the nation moved toward the final stretch.

A year after Cotton, a freshman congressman, announced his plan to challenge Democratic U.S. Sen. Mark Pryor, the two candidates and the outside groups involved in the race have already spent nearly $18 million, according to the nonprofit Sunlight Foundation, with much more to come.

The Democratic Senatorial Campaign Committee's new ad buy targets Cotton over his vote against funding pediatric research at Arkansas Children's Hospital. Democrats charge that the conservative Republican has failed to support federal programs important to Arkansas.

Cotton, meanwhile, over the weekend launched a six-figure television ad buy trying to cast Pryor as soft on immigration — an issue Republicans hope to focus on more during the August recess.

Cotton's campaign called the ad attacking him "patently false." Pryor's camp has accused Cotton's spot of misrepresenting his record on border security.

But the lines of attack in the campaign have long been set. Neither candidate faced a primary opponent in the race, which is seen as crucial to Republicans' effort to gain the six seats needed to win control of the Senate. Cotton and Republicans have repeatedly tried to tie Pryor to the president, who remains unpopular in Arkansas, and his signature health care law.

Pryor is "still a lockstep, rubber stamp vote for Barack Obama," Cotton said in an interview. "That's the one thing that hasn't changed about Mark Pryor over the last six years."

Pryor, a two term senator and the only Democrat in the state's congressional delegation, said the past year has only confirmed his portrayal of Cotton as out of step with the state on issues such as Medicare and the Farm Bill.

"His voting record continues to be what it is, and it's not good for Arkansas," Pryor told the AP earlier this week. "He really prioritizes the interests of his billionaire backers over the people of Arkansas. You see it all over the place."

Both candidates will campaign across the state during the August congressional recess.

"This just has all the hallmark signs of an epic battle to the end, whether television viewers want it or not," said Janine Parry, a political science professor at the University of Arkansas in Fayetteville.

Pryor said he thinks voters are "tired of this race already."

However, he said, "One thing you see is you see all the outside money coming in, but that really hasn't seemed to change the dynamics of the race." Pryor said.

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Elliott reported from Washington.

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