The record year was 2000, when 399,670 applications were denied, one-third of those presented, according to an analysis by the Migration Policy Institute, a nonpartisan research organization. More recent denial rates remain high, but have fallen from the peak because more immigrants have prepared with civics classes and immigrant advocates before applying to become citizens, researchers said.

In three recent cases in Florida, aspiring citizens thought their green cards entitled them to vote or register to vote before they were sworn in as Americans. When the immigrants reported their elections activities on their applications, not only were their naturalizations rejected, but they were also ordered to leave the country, according to their lawyer, Jeffrey Brauwerman.

In a current Florida case, a British-born businessman saw his naturalization derailed and was detained for deportation because he forgot to update his home address with the immigration agency, Mr. Brauwerman said. He was charged with ignoring a notice in which immigration examiners mistakenly accused him of a felony he had never committed.

In a case that drew Congressional attention this year in Illinois, Marin Turcinovic, an immigrant from Croatia, was twice denied citizenship because he did not show up at the immigration office to be fingerprinted. As his lawyer explained to no avail, Mr. Turcinovic was a quadriplegic, dependent on a ventilator and unable to leave his home.

Mr. Turcinovic died in April 2004 without becoming a citizen, creating an immigration crisis for his French widow, Corina, who had taken care of him. In January Representative Daniel Lipinski, Democrat of Illinois, presented a bill that halted her deportation.

Immigration officials say denials have increased in the last decade because naturalization applications are increasing. They note that approvals are rising as well. In 1996 naturalizations soared for the first time to more than one million, and they remained above 450,000 each year through 2007.

“Whenever we see a period when large numbers decide to apply, there tend to be larger numbers of people who are not ready or might not meet the requirements,” said Chris Rhatigan, a spokeswoman for Citizenship and Immigration Services.