Democrat Mark Kelly expanded his cash advantage over Sen. Martha McSally Martha Elizabeth McSallySenate Republicans face tough decision on replacing Ginsburg Democratic senator calls for eliminating filibuster, expanding Supreme Court if GOP fills vacancy What Senate Republicans have said about election-year Supreme Court vacancies MORE (R) in Arizona’s Senate race as the contest emerges as one of the most expensive down-ballot battles of the 2020 cycle.

Kelly, a gun control activist and former astronaut, raked in $11 million in the first quarter of the year, finishing the period with $19.7 million in the bank, his campaign told The Hill. McSally raised more than $6.3 million in the first three months of 2020 and has $10.2 million cash on hand.

Kelly also nearly tripled the number of donors McSally has garnered during the course of the Senate race, earning contributions from roughly 300,000 people compared with more than 110,000 for the incumbent.

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The Arizona Democrat relied heavily on small-dollar donors, with more than 90 percent of his contributions totaling less than $100 and the average donation amounting to $43.

The Kelly campaign declined to provide a statement on its fundraising haul.

The battle for McSally’s Senate seat is a major one for both Democrats and Republicans as Arizona emerges as a leading political battleground.

Sen. Kyrsten Sinema (D-Ariz.) was able to flip a Republican-held Senate seat in the 2018 midterms, and polls show former Vice President Joe Biden Joe BidenSenate Republicans face tough decision on replacing Ginsburg What Senate Republicans have said about election-year Supreme Court vacancies Biden says Ginsburg successor should be picked by candidate who wins on Nov. 3 MORE with a growing lead over President Trump Donald John TrumpObama calls on Senate not to fill Ginsburg's vacancy until after election Planned Parenthood: 'The fate of our rights' depends on Ginsburg replacement Progressive group to spend M in ad campaign on Supreme Court vacancy MORE in the traditionally red state.

Kelly has consistently held a polling lead over McSally, who was appointed to fill the vacancy left by the death of former Sen. John McCain John Sidney McCainMcSally says current Senate should vote on Trump nominee Say what you will about the presidential candidates, as long as it isn't 'They're too old' The electoral reality that the media ignores MORE (R). The two are running to serve out the final two years of McCain’s term.

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McSally’s seat is a crucial piece of both parties’ plans to lead the Senate: Reelecting her will be a major focus of the GOP’s effort to maintain its 53-47 majority, while Democrats will almost certainly have to get Kelly elected to capture the majority themselves.

The coronavirus pandemic has largely sidelined both campaigns, as well as candidates across the country, as people are encouraged to self-quarantine. Arizona's stay-at-home order was effective March 30, the second-to-last day of the first quarter.

McSally’s campaign expressed confidence that her work on relief packages related to COVID-19 will be recognized by Arizona voters.

“The last few weeks she has put politics aside and instead been working extremely hard to get relief to Arizonans during this pandemic by providing critical funding to hospitals, nonprofits, small businesses, and hard-working individuals and families,” her campaign said in a statement. “Senator McSally always puts Arizonans and their needs first, and voters continue to show growing support because they want someone in the U.S. Senate who leads by example.”

The Cook Political Report, a nonpartisan election handicapper, rates the Arizona Senate race as a “toss-up.”