This POLITICO article argues that Section 2 of the Fourteenth Amendment offers a way to penalize states that deny “or in any way abridge[]” the right to vote.  The federal government has never enforced this penalty clause.  If it did, Michael Linhorst theorizes it might do so as follows:

“A state that crosses the line would lose a percentage of its seats in the House of Representatives in proportion to how many voters it disenfranchises.  If a state abridges voting rights for, say, 10 percent of its eligible voters, that state would lose 10 percent of its representatives — and with fewer House seats, it would get fewer votes in the Electoral College, too.

Under the so-called penalty clause, it doesn’t matter how a state abridges the right to vote, or even why.  The framers of the constitutional amendment worried that they would not be able to predict all the creative ways that states would find to disenfranchise Black voters.  They designed the clause so that they wouldn’t have to.  ‘No matter what may be the ground of exclusion,’ Sen. Jacob Howard, a Republican from Michigan, explained in 1866, ‘whether a want of education, a want of property, a want of color, or a want of anything else, it is sufficient that the person is excluded from the category of voters, and the State loses representation in proportion.’”