Amy McGrath, Senate Majority Leader Mitch McConnell's leading Democratic opponent in the 2020 Kentucky Senate race, raised $10.7 million in the third quarter of 2019 after launching her campaign in July. McGrath, who narrowly lost a House race in 2018, has already proven her ability to draw large donations, as her campaign raised $2.5 million in the 24 hours after declaring her campaign.

McGrath's campaign received 299,000 contributions, with an average donation of $36, according to her campaign. Significantly, McGrath's third quarter haul is greater than that of several presidential candidates.

"Mitch McConnell has never faced an opponent like Amy McGrath. Not only does a new independent poll show the race tied, but more than a quarter-million grassroots donations-from all 120 Kentucky counties-have provided us a record-shattering first quarter of nearly $11 million to take down McConnell and his self-serving special interest allies. Change is coming," McGrath campaign manager Mark Nickolas said in a statement, referring to a poll of registered and likely voters by AARP showing McConnell ahead of McGrath by one percentage point.

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McConnell, who has represented Kentucky in the Senate since 1985, has easily dispatched Democratic challengers in previous elections. However, the senator is deeply unpopular in the state, and is the least popular senator in the country, with a disapproval rating of 50% in his home state, according to Morning Consult.

Although Kentucky is deeply Republican — President Trump won the state by 30 percentage points in 2016 — McConnell is viewed by many in Kentucky as a wholly political creature, a product of the "swamp" in Washington that Mr. Trump has promised to drain.

McGrath may try to peel away Kentucky voters who support Mr. Trump but are wary of McConnell. In her campaign launch video posted in July, McGrath blamed McConnell for effectively turning Washington into "a place where ideals go to die."

"Everything that's wrong in Washington had to start some place. How did it come to this, that even within our own families we can't talk to each other about the leaders of our country anymore without anger and blame? Well, that started with this man, who was elected a lifetime ago," McGrath said of McConnell.

However, McConnell has already been endorsed by Mr. Trump, and has worked with the president on key issues, such as confirming conservative judges to the federal bench.

The gubernatorial race in Kentucky may be a harbinger of McGrath's success or failure, as unpopular incumbent Republican Governor Matt Bevin faces moderate Democrat Andy Beshear in the November election. Bevin has pursued a strategy of embracing Mr. Trump and highlighting his relationship with the president in his campaign. If Bevin's support for Mr. Trump is unable to carry him to victory in the election, this could make it more difficult for McConnell to win his own Senate election by simply highlighting his ties to the president.

Sarah Ewall-Wice contributed to this report