Obama's speech has been hailed by the media because he is the first African-American nominee of a major political party and because it comes on the 45th anniversary of King's "I Have a Dream" speech.
Alveda King called her uncle "a man of great compassion, and a man of non-violence."
"He once said, 'The Negro cannot win as long as he is willing to sacrifice the lives of his children for comfort and safety,'" she added.
King said her uncle would understand that to include the destruction of unborn children.
"I know in my heart that if Uncle Martin were alive today, he would join with me in the greatest civil rights struggle of this generation - the recognition of the unborn child's basic right to life," she told LifeNews.com.
"My uncle Martin would agree that we cannot end poverty, hunger, or suffering by killing those who might suffer," she explained. "We cannot claim to guarantee equal rights if we deny the rights of the helpless. And we cannot feign ignorance of the fact that those who are torn apart, crushed, or left to die on an abortionist's table are just as human as we are."
"My uncle said that injustice anywhere is a threat to justice everywhere," Alveda continued.
"There is injustice in our land. We have turned being unwanted into a crime penalized by death. Today, I echo the words of Dr. Martin Luther King, Jr. - let freedom ring. From every mountain top, let freedom ring for all Americans, wanted and unwanted, born and unborn," she added.
Obama has repeatedly come under fire for his pro-abortion positions -- including a litmus test on Supreme Court judges to keep abortions legal another 35 years and wanting taxpayers to fund abortions.
He has also been criticized for opposing a bill in the Illinois legislature that would have provided medical care for newborns who survive botched abortions.