McGrath, a former Navy fighter pilot who had the backing of the national party establishment, will face the Republican majority leader in the Senate, Mitch McConnell, in November.
McGrath outperformed his Democratic opponent, State Representative Charles Booker, who enjoyed a belated wave of support when he emerged as a national voice during protests of police brutality and racial injustice and attracted support from progressives across the country.
A prolific fundraiser, McGrath was the pick of the Senate Democratic campaign arm and raised over $ 40 million for her campaign. He also enjoyed the backing of multiple unions and many Democrats, inside and outside the state, who were drawn to his military background. McGrath was the first woman to fly an F-18 in combat, and carried out more than 85 combat missions in Iraq and Afghanistan.
McGrath's supporters argued that his moderate stances were more aligned with the traditional Kentucky electorate than Booker's more progressive views. Booker, 35, supports the green New Deal (he is often referred to as the "Kentucky New Deal"), a universal basic income, and "Medicare for all." McGrath favors a public option and Medicare purchase for those 55 and older, rather than overhauling the US healthcare system with a single-payer program.
Booker, the youngest black legislator in Kentucky, received high-profile endorsement from Sens. Bernie Sanders and Elizabeth Warren, who voiced their support for McGrath last year, and New York Rep. Alexandria Ocasio-Cortez.
McConnell's Senate seat is rated "Likely Republican" by the Cook Political Report. The state voted for President Donald Trump by 30 points in 2016, and has not sent a Democrat to the Senate since Wendell Ford's reelection in 1992. McGrath now faces an uphill battle to beat McConnell, who is the Kentucky most time has served. senator.
In 2014, McConnell defeated another well-funded Democratic challenger, former Kentucky Secretary of State Alison Lundergan Grimes, who backed Booker in this primary, by more than 15 points.
Last year, McGrath launched his Senate nomination in a video highlighting his military background where he recalled writing a letter to McConnell when he was 13 and asked the senator to change a law that, at the time, prohibited women from becoming in fighter pilots. .
"He never answered," McGrath recalled. "Many times I have wondered, how many other people Mitch McConnell never took the time to answer, or even think about?"
McGrath's campaign has tried to portray McConnell as part of the Washington swamp who cares more about Wall Street and special interests than his own constituents. The McConnell campaign, which did not wait for the primaries to start attacking McGrath, called her an "extreme liberal" whose campaign will waste millions of dollars from Democrats.
Democrats, in turn, will be pleased that the Majority Leader has to divert his attention and his own fundraising apparatus to maintain his own seat, rather than training him more directly in a series of headlines in his caucus than they face difficult reelection races.
CNN's Alex Rogers contributed to this report.