Mothers of the Movement to Speak at Democratic National Convention

— -- Seven mothers united in grief over losing their children to gun violence or excessive use of force by police will put their message in the spotlight tonight when they deliver a speech at the Democratic National Convention in Philadelphia.

The group, Mothers of the Movement, includes Sybrina Fulton, the mother of Trayvon Martin, who was shot while walking in his Florida neighborhood in 2012, and Lezley McSpadden, the mother of Michael Brown, whose death in Ferguson, Missouri, in 2014 at the hands of a police officer set off nationwide protests.

The women previously met privately with Hillary Clinton and have endorsed her for president. They said Clinton calls them and writes when they are not on the campaign trail with her. In tonight's speech, they hope to deliver a powerful message.

"We must bring awareness," Gwen Carr, the mother of Eric Garner, who said, "I can't breathe," during a fatal altercation with New York police. "We must tell the people ... 'Don't wait till tragedy knock on your door. Start doing something now in the communities to help your neighborhood, help your communities.'"

The seven mothers sat down with "Good Morning America" co-anchor Robin Roberts in Philadelphia. They said Clinton is the candidate they believe can be held accountable to deal with gun violence and the use of excessive force by police in America.

"With the reform bill that she has in place, it implements it," said Maria Hamilton, the mother of Dontre Hamilton, who was killed by a Milwaukee police officer. "But we're going to hold her accountable for her actions with these bills and laws."

"She's the first presidential candidate that I'm aware of that has just said, 'This is a national crisis. It has to be dealt with,'" said Lucia McBath, whose son, Jordan Davis, was shot at a Florida gas station for playing music loudly. "Our communities and our families will never be safe unless we deal with gun violence in this country."

The mothers — also among them are Geneva Reed-Veal, the mother of Sandra Bland, who died in a Texas jail cell, and Cleopatra Cowley-Pendleton, the mother of Hadiya Pendleton, who was shot in a Chicago park with friends — will appear in a campaign video for Clinton that will debut tonight at the DNC.

Fulton said she and the other women are committed to a movement that is "much bigger than us."

"It's something that has been placed inside us that says, 'Listen, you can't help your son or daughters, but you certainly can help other children,'" she said.

"I never thought that I would be in this position," said Carr. "I was thrown into it. I never wanted this. I'd rather have my son."

Reed-Veal, whose daughter was jailed after being pulled over in a traffic stop, spoke about where she finds her motivation.

"It hurts, and then you feel like you have to get up and fight, you know," she said. "We know there is nobody in the world of my family that believes that Sandy killed herself."

The women said the conversation around gun violence and excessive use of force by police is now a little louder because of the recent deaths of police officers in Dallas and Baton Rouge in the wake of nationwide protests over police brutality.

"This is a bad time to be a good cop in this country," said Reed-Veal. "OK? We need to remember they have lives too."

McSpadden said, "Everybody gets caught up on the uniform, but under that uniform is somebody who belongs to somebody who really loves them."

McBath described the current racial situation in the U.S. as a "bursting at the seams."

"What you see happening to the police now, why they're being targeted, is because of all of the root of the systemic racism and the poverty and the way there's no relationship between the communities and the police and law enforcement," she said. "That is the root."

The women said their shared pain has made them very protective of one another. Their powerful message calls for leaders' accountability and for gun control legislation.

It's about "keeping guns out of the hands of the people who should not have guns," said McBath.