In an interview with the Washington Post, he said President Trump's rhetoric is the reason.
The Nationals beat the Houston Astros in Game 7 on Wednesday, earning the team's first-ever World Series title.
Among other issues, Doolittle told the Post he feels "very strongly" about Trump's "issues on race relations."
He pointed to the Central Park 5, the Fair Housing Act and Trump's comments in the wake of a 2017 white nationalist rally in Charlottesville, Virginia.
Doolittle said Trump's rhetoric has enabled and empowered racism and white supremacy.