First on CNN: Martin O'Malley to call for a voting rights constitutional amendment

Washington (CNN)Former Maryland Gov. Martin O'Malley will call for a constitutional amendment "to protect every citizen's right to vote" at a campaign event with black leaders in South Carolina on Tuesday, campaign aides tell CNN.

O'Malley, a Democratic candidate for president, will mark the 50th anniversary of the Voting Rights Act by arguing at a 20/20 Leaders of America meeting that the only way to protect voting rights against Republican efforts to "suppress the vote" is to guarantee the right with a constitutional amendment.

After the event, O'Malley will send an email to his supporters asking them to stand behind his amendment push.

"Last year, Republican state legislators in 29 states introduced more than 80 restrictive bills to require a photo ID, make voter registration harder, or reduce early voting," O'Malley writes in the email provided to CNN. "We know why they're doing this: because Americans without a photo ID are disproportionately low-income, disabled, minority — and Democratic."