The groups, which rallied around Harris since she entered the race in July, have helped raise more than $20 million for her campaign, a Unite for America press release said.
"What is essential to me is getting people motivated to vote, and ensuring everyone makes a plan not only for themselves, but for their families and friends," Winfrey, a businesswoman, philanthropist and former TV host, said in a statement.
In a recent Reuters poll, Harris led her Republican opponent Donald Trump 47% to 42%.
Harris was ahead in the battleground states of Michigan, Pennsylvania, Wisconsin, Nevada and North Carolina, while Trump had greater support in Georgia, according to polling averages from aggregator FiveThirtyEight. They are tied in Arizona.
Trump on Thursday will participate in a Washington event on fighting antisemitism and address the Israeli-American Council Summit.
Hours after President Joe Biden dropped out and endorsed Harris to be the Democratic candidate, 90,000 Black women joined a call
held by Win with Black Women that raised $1.5 million in three hours, the press release said.
In August, Winfrey spoke at the Democratic National Convention in Chicago to press the case for Harris.
"I'm calling on all you independents and all you undecided," Winfrey said then, describing herself as an independent voter. "Decency and respect are on the ballot in 2024."