A Democrat-aligned political action committee has debuted its first ad in the critical North Carolina Senate race, targeting GOP Sen. Richard Burr Richard Mauze BurrRep. Mark Walker says he's been contacted about Liberty University vacancy Overnight Defense: Trump rejects major cut to military health care | Senate report says Trump campaign's Russia contacts posed 'grave' threat Senate report describes closer ties between 2016 Trump campaign, Russia MORE over Medicare.

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End Citizens United PAC, a group seeking to overturn the 2010 Supreme Court ruling in Citizens United v. Federal Elections Commission, knocks Burr for a 2007 procedural vote.

Burr voted against taking up legislation that would have allowed Medicare to negotiate with pharmaceutical companies over the drugs it purchases. Republicans at the time of the 2007 vote argued that private insurers were already negotiating price cuts for people on Medicare.

The PAC links Burr's vote to campaign donations he's received from the pharmaceutical industry.

“Senator Burr voted to block Medicare from negotiating lower drug prices,” the ad’s narrator says. “And has taken $1.4 million in campaign contributions from the pharmaceutical industry. More campaign cash for him. Higher costs for us.”

The ad, shared first with The Hill, is part of a $900,000 TV and digital ad buy that will run in Charlotte until Nov. 4. The group announced this buy earlier this month, which is part of a $5 million ad blitz in key Senate and House races in the final weeks of the election.

The North Carolina Senate race has become of the country’s marquee races this cycle, and the debate over Medicare has yielded some of the biggest clashes between Burr and his opponent, former state Rep. Deborah Ross (D).

Ross has attacked Burr for wanting to "privatize" Medicare. She has pointed to legislation he proposed in 2012 that would have raised the eligibility age for Medicare and allowed private firms to bid to offer Medicare services.

Burr told McClatchy last month that he wouldn’t pursue a vote on his proposal but later told Morning Consult that he stands by his plan.

“I’ve never said that I divorce myself from the proposal I’ve put out there. I stand by everything legislative I’ve done or proposed,” Burr told Morning Consult in September.

The race in the Tar Heel State could determine which party controls the Senate next year. Ross started to close the gap, but Burr has rebounded in the most recent polls. The GOP senator holds a 3.2-point edge, according to RealClearPolitics polling average.