"I think those numbers speak for themselves. Good grief," said Jim Pederson, a former state Democratic Party chair who ran for Senate in 2006.

“The guy is a very personable, sharp guy. And he's made a lot of friends nationwide, and I think he's tapped into that network,” Pederson said, adding that Arizonans were "wrapped up into his enthusiasm."

Republicans have taken notice as well.

"It’s not necessarily that Sen. McSally is doing poorly. It is that Mark Kelly is doing spectacularly,” said Paul Bentz, a Republican strategist in Arizona. “He's doing above and beyond, I think, what anybody would have anticipated when it comes to fundraising.

“She has the power of incumbency, but he has definitely caught up on many, if not all, other campaign metrics,” Bentz added.

Arizona is a critical state in next year's elections, with the presidency and Senate majority up for grabs. Democrats won their first Senate race in three decades in the emerging swing state last year, when Democrat Kyrsten Sinema beat McSally to capture the seat held by GOP Sen. Jeff Flake, a conservative Trump opponent who retired as he drew the president's ire.

Sen. Martha McSally. | Mark Wilson/Getty Images

After her defeat, Doug Ducey, the state's Republican governor, appointed McSally to fill a vacancy for the state's other Senate seat, which was held by Republican John McCain until his death last year. McSally is running again to complete the remaining two years on McCain's unexpired term for 2021 and 2022.

Several advantages have helped Kelly stockpile cash. Kelly was the first major Senate candidate to announce a bid, joining the race in February with built-in name ID and connections to donors through his work with a gun control organization he founded with Giffords, the former congresswoman who was shot in 2011. The national party has rallied behind Kelly, and he’s avoided even a whiff of a primary challenge.

Kelly has also invested in building a campaign to last. He has spent more than $700,000 on Facebook ads this year, which helps build a list of small-dollar donors that will benefit him down the road, according to data compiled by ACRONYM, a progressive digital organization. Last quarter, Kelly spent $1.9 million, with $420,000 of that earmarked for digital advertising and more than $640,000 for direct mail services, both designed to help raise new money.

In the third quarter, more than half of his money raised — $2.9 million — came in unitemized donations that are less than $200. He also raised nearly $1.3 million from donors who gave more than $1,000, including more than $667,000 from max-out donors.

“This campaign is powered by grassroots supporters who are chipping in what they can, when they can because they support Mark’s mission to be an independent voice for Arizona," said Jacob Peters, a spokesperson for Kelly's campaign.

The online money, in particular, is what has set Kelly apart early in the cycle.

“I think they've smartly rejected the dumb conventional wisdom out there that only super progressives can raise small dollars online,” said Andy Barr, a Democratic strategist who worked on the 2012 Arizona Senate race.

Republicans acknowledge Kelly’s fundraising prowess but say they are not caught off guard by it. The party expected him to raise significant sums after being recruited to join the race and praise McSally’s efforts to keep pace — especially for someone in the unique position of preparing for reelection while also adjusting to her first nine months in the Senate.

“She's doing great, doing the things a candidate and incumbent need to do,” said Barrett Marson, a veteran Republican strategist who heads a super PAC that backs McSally. “Obviously, Mark Kelly is raising a lot of money. She can't control that. She can only control what she does, and Sen. McSally is doing the right things.”

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McSally has also proven to be a strong online fundraiser, despite the party’s broader struggles keeping pace in small dollars. She raised $1.2 million in unitemized donations under $200, and her campaign said nearly 100,000 donors have given $100 or less. McSally spent nearly $350,000 on online fundraising, plus another $60,000 in fundraising list rentals, in the third quarter of the year, a larger investment than most other Republicans. Her campaign has lagged on Facebook, however, and has spent only $35,000 on the platform, according to the data from ACRONYM.

“Martha’s consistently strong fundraising numbers prove that Arizonans are unified in their support for her to keep fighting for them in the U.S. Senate,” said her general consultant, Terry Nelson, in a statement.

Dan Eberhart, a major GOP donor, warned that McSally would likely be outspent without “major support” from outside groups like the National Republican Senatorial Committee and Senate Leadership Fund, a super PAC aligned with GOP leadership. But Republican groups recognize McSally is critical to a GOP majority and are planning to play big in Arizona.

“We think Martha McSally is one of the most compelling leaders in America today, which is why Arizona was one of our largest investments in 2018 — and we expect it will be the same in 2020,” said Jack Pandol, spokesperson for the Senate Leadership Fund.