“You’re only as good as your next election,” she added.

The Pew study focused on the 2008 and 2010 elections, the most recent ones for which comprehensive data were available.

The study also found wide variation in how easy registering to vote can be. North Dakota does not even require it, and Alabama and Kansas reported rejecting less than 0.05 percent of registration applications in 2008. But Pennsylvania and Indiana each rejected more than half of the registration applications they received in 2010.

On Election Day, the voting experience can also vary. The 10 states with the shortest waiting times at the polls in 2008 averaged six minutes, the study found. In South Carolina, the wait was more than an hour.

The shift to voting by mail, which now accounts for some 20 percent of all ballots cast, tends to eliminate lines. But it has also produced new problems, especially in places where mail voting has soared because the state does not require an excuse or a new ballot request for each election. Arizona and California, where voting by mail is commonplace, had among the highest rates of problems with voter registration and absentee ballots.

In 2010, California rejected absentee ballots 0.7 percent of the time, a higher rate than any other state.

Dean C. Logan, the registrar for Los Angeles County, said the rate was partly a byproduct of the popularity of voting by mail in California and partly a function of how the state defines rejected ballots. Its definition includes ballots that voters requested but that the Postal Service returned to election officials as undeliverable.

“Voter behavior is changing and evolving,” Mr. Logan said. Young people do not sign their names as consistently as older ones, he said, and mail delivery is becoming less reliable.