Rep. Matt Salmon is "taking a serious look" at a primary run against Sen. John McCain in 2016, a source close to the representative told National Journal on Thursday.

The statement is the strongest indication yet that the tea-party-backed politician from Arizona might leave his House seat to challenge the five-term incumbent.

The source, who requested anonymity to speak candidly about Salmon's possible plans, said the congressman has been "gratified by the strong level of support" he's seen from supporters, and would likely make a decision by early August.

If Salmon runs, it would begin what would likely be the election cycle's most closely watched primary, pitting the former GOP presidential nominee against a sitting lawmaker with the strong support of local and national conservative groups. Adding to the intrigue is that either McCain or Salmon could face a difficult foe in the general election, after Democratic Rep. Ann Kirkpatrick surprisingly announced this week that she would run for Senate.

The New York Times this week identified Kirkpatrick as McCain's first "seasoned Democratic challenger" in his five Senate reelection bids, potentially setting him up against both his toughest Democratic opponent and primary challenger yet, at the same time.

McCain, who is 78, announced his intentions to run for reelection in early April, noting at the time that he was "ready for whatever comes," and that "no success in [his] life has ever come without a good fight."

McCain spokesman Brian Rogers said in a statement Thursday that the senator "has been through a tough fight or two—in politics and life. He's got a remarkable record of fighting for Arizona and America, and he'll be well-prepared for any challenge that comes."

"Congressman Salmon remains focused on representing the constituents of Arizona's Fifth District, not on making plans to seek reelection or any other office," said Salmon's congressional spokesman, Tristan Daedalus.

Salmon has long been regarded by groups such as the Senate Conservatives Fund and FreedomWorks as their best shot at unseating McCain after failing to topple him in 2010. They labeled McCain, a long-time antagonist of some conservative causes, their top target at the beginning of the 2016 cycle, with one strategist suggesting a second loss against him could mean that "primary challenges are dead."

Club for Growth President David McIntosh said in February that his group was also watching the race, and would consider supporting Salmon, who they endorsed in his congressional campaign.

Many of these groups have indicated that since Rep. David Schweikert turned down the race that their options are nearly Salmon or bust. Given McCain's sizable war chest—he has more than $3.6 million in his reelection account—they'd be hesitant to go all in for a second-tier candidate such as state Sen. Kelli Ward, who also is considering the entering the primary.

Meanwhile, Kirkpatrick has been regarded as a top recruit for Democrats, having proven her ability to win an uphill reelection battle in a swing district that Mitt Romney carried in 2012. Her decision to join the race this week took many by surprise, including fellow Democratic Rep. Kyrsten Sinema, who was rumored to be considering the race.

McCain remains the heavy favorite going into either a primary or a general election, but recent polling by Democratic firm Public Policy Polling showed his favorability low among Republicans, especially those who considered themselves "very conservative." Salmon's allies say those numbers reflect what they see in private polling as well.

A source close to Salmon said that while the Kirkpatrick news was a happy surprise, "they probably did a lot of polling, and they're probably seeing the same things we are."

This story has been updated with a statement from Salmon's congressional office.

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