Such a law, the suit alleges, is racist in its origins and is biased in its effect, disenfranchising roughly 15 percent of Alabama’s black voting age population, compared with fewer than 5 percent of whites. The suit argues that bias lies in the crimes generally chosen as involving moral turpitude — there are multiple interpretations of the crimes in that category — and in the requirement that fines and restitution be paid before the right to vote can be granted again, a condition that falls harder on the poor.

As the country’s incarceration rate grew over the past three decades, according to the Sentencing Project, a Washington-based nonprofit group, laws like the one in Alabama left nearly six million people ineligible to vote by 2010. But over the past 15 years, said Marc Mauer, the executive director of the Sentencing Project, there has been a broad and largely successful movement against these laws.

“The vast majority of the reforms that have taken place have been toward expanding the electorate, cutting back on longstanding categories of disenfranchisement and/or making the process more transparent,” Mr. Mauer said, speaking of recent activity along these lines in California, Maryland and Virginia.

Still, there are outspoken opponents of this trend, who argue that the ability of felons to vote again, even if attainable, should not be automatic, to reflect the seriousness of criminal conduct. Eleven states prohibit some or all felons from voting permanently, barring a pardon or other government action. Alabama, like several other states, allows some felons to regain voting eligibility; in four states — Florida, Iowa, Kentucky and Virginia — all of those convicted of felonies permanently lose their right to vote. In Virginia, Gov. Terry McAuliffe, a Democrat, this year issued a sweeping executive order that restored voting rights to 206,000 ex-offenders, an order that was struck down in July by the Virginia Supreme Court.

While the lawsuit filed in Alabama on Monday challenges felony disenfranchisement generally, its focus is on the state’s “moral turpitude” clause, a phrase that is not formally defined in law. Instead, it leaves the decision of who gets to vote, essentially, up to the local registrar.