Retired astronaut Mark Kelly leads Sen. Martha McSally Martha Elizabeth McSallyTrump supporters chant 'Fill that seat' at North Carolina rally Momentum growing among Republicans for Supreme Court vote before Election Day Vulnerable GOP incumbents embrace filling Supreme Court seat this year MORE (R) by 9 points in Arizona, one of the states at the heart of the battle for control of the Senate in 2020.

A new Arizona Public Opinion Pulse survey conducted by the Phoenix-based nonpartisan polling firm OH Predictive Insights shows Kelly leading McSally by a 51 percent to 42 percent margin.

The company's last survey found Kelly leading by a 7-point margin, 49 percent to 42 percent.

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Kelly, a first-time candidate and the husband of former Rep. Gabrielle Giffords (D-Ariz.), leads McSally by 10 points in Maricopa County, which accounts for the vast majority of Arizona's vote. Only one candidate in recent history, former Superintendent of Public Instruction Diane Douglas (R) in 2015, has won statewide election in Arizona without carrying Maricopa County.

Kelly also leads by a huge margin among independent voters, taking two-thirds among those who side with neither Democrats nor Republicans. McSally, who lost a close race for Arizona's other U.S. Senate seat in 2018 before being appointed to fill two years of the late Sen. John McCain John Sidney McCainMomentum growing among Republicans for Supreme Court vote before Election Day McConnell urges GOP senators to 'keep your powder dry' on Supreme Court vacancy McSally says current Senate should vote on Trump nominee MORE's (R) seat, takes just under a quarter of independent voters.

The same poll found former Vice President Joe Biden Joe BidenMomentum growing among Republicans for Supreme Court vote before Election Day Trump expects to nominate woman to replace Ginsburg next week Video of Lindsey Graham arguing against nominating a Supreme Court justice in an election year goes viral MORE leading President Trump Donald John TrumpUS reimposes UN sanctions on Iran amid increasing tensions Jeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Trump supporters chant 'Fill that seat' at North Carolina rally MORE among Arizona voters by a 52 percent to 43 percent margin. Biden is hoping to become the first Democratic presidential candidate since Bill Clinton William (Bill) Jefferson ClintonDolly Parton remembers Ginsburg: 'Her voice was soft but her message rang loud' Sunday shows preview: Justice Ginsburg dies, sparking partisan battle over vacancy before election Calls grow for Biden to expand election map in final sprint MORE to win Arizona's electoral votes.

"McSally's path to victory is difficult, but not impossible," said Mike Noble, OH Predictive Insight's chief of research. "She needs to expand her base outside of Trump's base of support by winning over women, independents, Latinos and suburban voters in Maricopa County."

Arizona is a must-win state for Democrats seeking to reclaim control of the Senate, where Republicans hold 53 seats, along with states like Colorado, Maine and North Carolina.

Kelly has been the party's all-star recruit. He pulled in $11 million in the first three months of the year, his campaign said Tuesday, and held $19.7 million in the bank.

McSally is no fundraising slouch herself: Her campaign said she raised $6.3 million and had $10.2 million on hand, figures that put her among the GOP's best fundraisers.