The trend will probably result in more uncounted votes, and it increases the potential for fraud. While fraud in voting by mail is far less common than innocent errors, it is vastly more prevalent than the in-person voting fraud that has attracted far more attention, election administrators say.

In Florida, absentee-ballot scandals seem to arrive like clockwork around election time. Before this year’s primary, for example, a woman in Hialeah was charged with forging an elderly voter’s signature, a felony, and possessing 31 completed absentee ballots, 29 more than allowed under a local law.

The flaws of absentee voting raise questions about the most elementary promises of democracy. “The right to have one’s vote counted is as important as the act of voting itself,” Justice Paul H. Anderson of the Minnesota Supreme Court wrote while considering disputed absentee ballots in the close 2008 Senate election between Al Franken and Norm Coleman.

Voting by mail is now common enough and problematic enough that election experts say there have been multiple elections in which no one can say with confidence which candidate was the deserved winner. The list includes the 2000 presidential election, in which problems with absentee ballots in Florida were a little-noticed footnote to other issues.

In the last presidential election, 35.5 million voters requested absentee ballots, but only 27.9 million absentee votes were counted, according to a study by Charles Stewart III, a political scientist at the Massachusetts Institute of Technology. He calculated that 3.9 million ballots requested by voters never reached them; that another 2.9 million ballots received by voters did not make it back to election officials; and that election officials rejected 800,000 ballots. That suggests an overall failure rate of as much as 21 percent.

Some voters presumably decided not to vote after receiving ballots, but Mr. Stewart said many others most likely tried to vote and were thwarted. “If 20 percent, or even 10 percent, of voters who stood in line on Election Day were turned away,” he wrote in the study, published in The Journal of Legislation and Public Policy, “there would be national outrage.”

The list of very close elections includes the 2008 Senate race in Minnesota, in which Mr. Franken’s victory over Mr. Coleman, the Republican incumbent, helped give Democrats the 60 votes in the Senate needed to pass President Obama’s health care bill. Mr. Franken won by 312 votes, while state officials rejected 12,000 absentee ballots. Recent primary elections in New York involving Republican state senators who had voted to allow same-sex marriage also hinged on absentee ballots.