In all, nine states have passed measures making it harder to vote since the beginning of 2013. Most have to do with voter ID laws. Other states are considering mandating proof of citizenship, like a birth certificate or a passport, after a federal court judge recently upheld such laws passed in Arizona and Kansas. Because many poor people do not have either and because documents can take time and money to obtain, Democrats say the ruling makes it far more difficult for people to register.

Voting experts say the impact of the measures on voter turnout remains unclear. Many of the measures have yet to take effect, and a few will not start until 2016. But at a time when Democrats are on the defensive over the Affordable Care Act and are being significantly outspent by conservative donors like the Koch brothers, the changes pose another potential hurdle for Democratic candidates this year.

Republicans defend the measures, saying Democrats are overstating their impact for partisan reasons. The new rules, Republicans say, help prevent fraud, save money and bring greater uniformity to a patchwork election system.

“We think they’re stoking these things for political gain,” said Alex M. Triantafilou, the chairman of the Hamilton County Republican Party in Ohio. “We think there’s an effort here to rally the Democratic base in a year that they otherwise wouldn’t be rallying.”

Democrats and other critics of the laws say that in the face of shifting demographics, Republicans are trying to alter the rules and shape the electorate in their favor. Those most affected by the restrictions are minorities and the urban poor, who tend to vote Democratic.