GOP hopes for winning back the Senate rest on Arkansas, where Rep. Tom CottonThomas (Tom) Bryant CottonGOP looks for Plan B after failure of immigration measures Senate rejects Trump immigration plan Our intelligence chiefs just want to tell the truth about national security MORE (R) is running to unseat Sen. Mark PryorMark Lunsford PryorMedicaid rollback looms for GOP senators in 2020 Cotton pitches anti-Democrat message to SC delegation Ex-Sen. Kay Hagan joins lobby firm MORE, widely seen as the most vulnerable Democrat running in 2014. [WATCH VIDEO]

A Cotton victory over Pryor would open the door for Republicans to win the six seats they need to control the upper chamber.

But if Pryor survives, the GOP’s hopes of taking over the Senate will all but evaporate.

“There really is no path to the majority that doesn't include Arkansas,” one national Republican strategist told The Hill. “It's hard to imagine that we'd lose that seat and win in North Carolina, Louisiana and Alaska. That'd be tough.”

Those states are the homes of the three other most vulnerable Democrats running next year — but none are seen to be in as much trouble as Pryor.

President Obama won just 37 percent of the state’s vote in 2012, and Pryor is the last Democrat left standing in his state’s congressional delegation. Sen. Blanche Lincoln (D-Ark.) lost overwhelmingly in 2010 to then-Rep. John BoozmanJohn Nichols BoozmanSenate Republicans call on Trump to preserve NAFTA Dems go on the attack during EPA chief's hearing Bipartisan group of senators ask Trump to fund broadband in infrastructure plan MORE (R-Ark.) months after voting, like Pryor, for ObamaCare’s passage.

A series of polls released in recent days suggests a close race between Pryor and Cotton, who formally launched his campaign Tuesday.

Yet all show Pryor topping out with 43 percent support, a dangerously low number for an incumbent. Republicans are confident in Cotton and appear to already be sizing up Pryor’s office. But the incumbent says he’s not scared of a challenge, noting he’s been written off before.

“The most vulnerable label — they talked about that in 2002 when I ran; they said I couldn't win,” Pryor told The Hill in an interview.

“In 2008, I was a one-term wonder, and then I won with almost 80 percent of the vote,” he added. People who say he’s the most vulnerable Senate Democrat running for reelection “do not understand Arkansas politics,” Pryor said.

“We've heard that from Washington for years. It doesn't faze me. It doesn't bother me," he said. "If I was so vulnerable, why would they be spending all this outside money against me?” 

Republicans say Pryor should be seen as vulnerable.

“An incumbent U.S. senator whose reelect number is between 35 and 43 percent is in deep trouble,” said Arkansas Republican Party spokesman David Ray. “There's no way you can church that up. There's no way you can sugarcoat that.

“There's no question this will be a hard-fought race,” Ray continued. “It's still early in the race. It's yet to really take shape, but the preliminary signs show we have an excellent opportunity, and we're going to have the best possible candidate.”

Republicans say the unpopularity of Democrats up and down the ballot and Cotton’s impressive resume as a former Army ranger will help him win comfortably.

Democrats predict Pryor’s independent streak and strong family name, coupled with Cotton’s very conservative voting record and relative inexperience with running tough races, will help Pryor pull off a narrow win.

Pryor, who in the interview acknowledged the race would be a hard one, is already airing ads blasting Cotton as a political opportunist out of step with Arkansas voters. Arkansas’s Democratic Party also has launched a website attacking Cotton’s voting record.

Outside groups have already spent more than $1.2 million attacking Pryor. Conservatives have tied him to Obama, while gun control groups have criticized his vote against background check legislation. Nonpartisan observers predict a close race.

“This race will get framed either as a referendum on Mark Pryor as Barack ObamaBarack Hussein ObamaOvernight Energy: Dems ask Pruitt to justify first-class travel | Obama EPA chief says reg rollback won't stand | Ex-adviser expects Trump to eventually rejoin Paris accord Overnight Regulation: Trump to take steps to ban bump stocks | Trump eases rules on insurance sold outside of ObamaCare | FCC to officially rescind net neutrality Thursday | Obama EPA chief: Reg rollback won't stand Ex-US ambassador: Mueller is the one who is tough on Russia MORE, or a referendum on whether Cotton is too extreme to get elected,” said Roby Brock, the editor of the state-based political website Talk Business.

He expects the race to go down to the wire.

“The negatives are what's going to win out,” he said.