The day started out strong for Booker, who edged out McGrath for the Fayette Co. primary vote, and won in a landslide for the primary vote in Jefferson Co, propelling him to take the lead around 10:30 a.m.
But a late surge from McGrath locked up the win, regaining the lead with about 89-percent of precincts to report, and she'll move forward to November's election.
Although voting in the state ended on June 23, McGrath was declared the victor a week later due to the race's tight margins and a surge of mail-in ballots.
Despite having the backing of national Democrats and a formidable war chest, McGrath found herself in an increasingly competitive battle for the nomination with Booker, 35, the youngest black state lawmaker in Kentucky from the Louisville-area. The tally had been delayed because of the large number of mail-in votes that needed to be counted of election day.
McConnell, who is seeking a seventh term, easily prevailed in the Republican primary last week.
The Louisville Courier-Journal chided national Democrats for the early support of McGrath, who struggled with her campaign launch last summer when she said she would have voted to confirm Supreme Court Justice Brett Kavanaugh before nearly immediately reversing course and saying she wouldn't.
But Booker ended up mounting a far tougher challenge than expected, fueled by momentum from his vocal support for protests against the shooting death by police of a Louisville woman.
"Extreme Amy McGrath is lucky to have gotten out of the primary with a victory, but her reputation sustained significant damage all across Kentucky", Cooksey said. He joined demonstrations in his hometown of Louisville to demand justice for Breonna Taylor, who was fatally shot by Louisville police in her own home. "McGrath is just another tool of the Washington Democratic establishment who has no idea what matters most to Kentuckians". She supports adding a public health insurance option as part of the Obama-era Affordable Care Act and supports expanded access to Medicare for people 55 and older.
She portrays McConnell as an overly partisan, Washington insider who exemplifies what's wrong with national politics. McConnell's campaign has said McGrath is too liberal for Kentucky, on issues from abortion to border security. McConnell also plays up his Senate leadership role and his ability to steer federal money back to the Bluegrass State.
Trump could turn into a focal point in the Senate race.
"The mission to defeat Mitch McConnell and defend our democracy goes on", she said.