LOS ANGELES — More than 700,000 immigrants are waiting on applications to become U.S. citizens, a process that once typically took about six months but has stretched to more than two years in some places under the administration of President Donald Trump.The long wait times have prompted some immigrant advocates to ask whether the delays are aimed at keeping anti-Trump voters from casting ballots in elections. "People are motivated to participate, and they’re being frustrated from being able to participate in the elections they’re excited about," said Manuel Pastor, director of the University of Southern California’s Center for the Study of Immigrant Integration.The number of immigrants aspiring to become U.S. citizens surged during 2016, jumping 27 percent from a year earlier as Trump made cracking down on immigration a central theme of his presidential campaign. At first, the federal government kept up with the applications, but then the wait grew.Backlogs are nothing new in the U.S. immigration system. It often takes years to receive asylum or to be deported. But naturalization — the final step to become an American citizen, obtain a U.S. passport and receive voting rights — had not been subject to such delays in recent years.Now the average wait time for officials to decide on applications is more than 10 months. It takes up to 22 months in Atlanta and as long as 26 months in parts of Texas, according to official estimates.Trump tweeted on Thursday that Central American migrants headed north in a U.S.-bound caravan should return home and can apply for American citizenship if they wish. "Go back to your Country and if you want, apply for citizenship like millions of others are doing!" he posted as thousands continued their trek through Mexico. But immigrants generally must be legal permanent residents of the United States to apply for citizenship, and getting a green card can take years — if a person even qualifies for one. U.S. Citizenship and Immigration Services said the longer waits to naturalize are because of the surge in applications, not slower processing. The agency decided 850,000 cases in 2017, up 8 percent from a year before. Despite "a record and unprecedented" spike in applications, the agency is operating more efficiently and effectively and "outperforming itself," spokesman Michael Bars said in a statement. To become an American citizen, immigrants must hold green cards for at least three years, demonstrate good moral character and pass English and civics tests.Migrant caravan takes a break: Thousands of Central American migrants took a break Sunday on their long journey through southern Mexico, but vowed to press ahead toward the U.S. border roughly 1,000 miles away, with Mexican government agencies seeming to waver between helping and hindering.