Amy McGrath, a Democratic challenger for Senate Majority Leader Mitch McConnell Addison (Mitch) Mitchell McConnellTrump 'no longer angry' at Romney because of Supreme Court stance On The Money: Anxious Democrats push for vote on COVID-19 aid | Pelosi, Mnuchin ready to restart talks | Weekly jobless claims increase | Senate treads close to shutdown deadline The Hill's Campaign Report: Trump faces backlash after not committing to peaceful transition of power MORE's (R-Ky.) seat in 2020, raised $2.5 million on the first day of her campaign.

McGrath's campaign told NBC News that more than $1 million came in in the first 5 1/2 hours after the campaign launched on Tuesday.

McGrath campaign manager Mark Nickolas told NBC News that the $2.5 million haul came from online donations, with the average contribution being $36.15.

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Nickolas said that the amount raised was the largest for the first 24 hours of a Senate campaign.

McGrath, who previously lost a congressional a bid in 2018, announced her candidacy for the Senate in a three-minute campaign video.

She was one of the most closely watched Democratic House contenders in the 2018 midterm elections, narrowly losing her race to incumbent Rep. Andy Barr (R) in Kentucky's 6th Congressional District.

The fundraising haul comes on the heels of a series of strong second-quarter fundraising numbers from Democrats looking to unseat incumbent Senate Republicans.

Sen. Susan Collins's (R-Maine) challenger, state Speaker Sara Gideon (D), raised more than $1 million in the first week of her campaign, while Sen. Lindsey Graham's (R-S.C.) Democratic challenger, Jaime Harrison, brought in $1.5 million since launching his campaign just a month ago.

Democrat Mark Kelly, who is running to unseat Sen. Martha McSally Martha Elizabeth McSallyThe Hill's Campaign Report: Presidential polls tighten weeks out from Election Day Mark Kelly: Arizona Senate race winner should be sworn in 'promptly' New ABC/WaPost poll finds Trump edging Biden in Arizona, Florida MORE (R-Ariz.), raised $4.2 million in the second quarter, outpacing his first-quarter haul.

McConnell is set on maintaining his party's control of the chamber, in which they have a 53-47 majority.

He won easily won reelection to his sixth term in 2014 by nearly 16 points.