Kemp - who, as Georgia secretary of state, is also in charge of elections and voter registration - faced backlash after it was reported his office had put more than 53,000 voter applications on hold, almost 70% of which are from African-Americans, because they failed to clear the state's controversial "exact match" standard.
On Sunday, President Trump is rallying for Kemp.
More high-profile visits are planned, with former President Barack Obama coming to Atlanta Friday for Abrams and President Donald Trump campaigning Sunday for Kemp.
She has declared she would use the gubernatorial role to expand healthcare, increase spending on public education and reforming criminal justice.
In an apparent reference to the controversy, Winfrey said, "Every single one of us - every single one of us - has the same power at the polls, and every single one of us has something that, if done in numbers too big to tamper with" - she paused for resounding, prolonged applause by the audience - "cannot be suppressed and can not be denied".
Mike Pence, who was in town to stump for Kemp said, "I heard Oprah was in town today and I heard Will Ferrell was going door-to-door the other day. I am not trying to test any water, don't want to go in those waters".
Ms Winfrey, who rarely makes political endorsements, drew cheers when she said she's a registered independent who was not in Georgia at anyone's request.More news: Trump Sends 5,200 Troops To Mexico Border As Caravan Advances
She added that Abrams is "standing strong for the values that matter to me and the values that matter to Georgians all over this state". "I don't want to go in those waters".
"I see that same potential in Stacey Abrams", the vile message continues.
Friday's ruling required Kemp to allow 3,000 of those people, whose voting registration was placed in "pending" status due to questions about their US citizenship, to vote in the November 6 USA midterm election if they present proof of citizenship at the polls.
As a black woman, Winfrey noted her kinship to two groups historically denied ballot access in the United States. She recalled generations of black Americans who faced "lynching ... oppression ... suppression", and declared that "their blood has seeped into my DNA" and forced her to the polls.
More than 1.5 million Georgians already have cast ballots.
Videos uploaded to Ms Winfrey's and Ms Abrams' social media showed the television personality knocking on doors canvassing for the hopeful governor.
"While we cannot comment on the specifics of an ongoing investigation, I can confirm that the Democratic Party of Georgia is under investigation for possible cyber crimes", Candice Broce, Kemp's press secretary, said in a statement.