"We cannot let Barack Obama's legacy fall into Donald Trump's hands," Democratic front-runner Hillary Clinton said.
"Let's not endanger the promise, the potential, the dream of our country by giving in to these voices of hatred," the Senator and former Secretary of State said on Sunday night.
She was addressing a National Association for the Advancement of Coloured People (NAACP) dinner, where she contrasted Obama and Republican hopeful Trump.
In her speech in Detroit, she mentioned Trump's ties to the "insidious birther movement" and his initial struggle to disavow former Ku Klux Klan leader David Duke, The Washington Post reported.
Clinton said bringing the country together was more urgent than ever with Obama's presidency ending.
The NAACP dinner was her first public event in Michigan since losing the state's March 8 primary by a percentage and a half to rival Bernie Sanders.
Attendee and Clinton supporter Shawn Thomas said he thought she struck the right tone on Sunday in going after Trump but he urged caution.
"Everybody who goes after Trump just gets dragged into the mud," he said.
The NAACP is an African-American civil rights organisation in the US, formed in 1909.