TUCSON, Ariz. - They shuffle into the courtroom in shackles, still in the dust-covered clothes and shoes they wore crossing the desert into the United States from Mexico.

The 70 illegal immigrants, mostly men in their 20s and 30s, fill the 16-seat jury box and seven rows of wooden benches normally reserved for the public in Tucson's gleaming federal courthouse.

In only an hour or so, the dozens of immigrants will agree to plead guilty and be sentenced in a process that could play out for months for most federal defendants.

The scene offers a window into a federal immigration enforcement effort that is pushing the limits of the U.S. justice system, overwhelming federal judges, and swelling the number of Latinos sent to prison.

Expedited court hearings along the border are a major force driving a seismic demographic shift in who is being sent to federal prison. Statistics released last week revealed that Hispanics now make up nearly half of all people sentenced for federal felony crimes, a number swollen by immigration offenses. Hispanics last year made up 16 percent of the total U.S. population.

Sentences for felony immigration crimes, which include illegal crossing as well as other crimes such as alien smuggling, accounted for about 87 percent of the increase in the number of Hispanics sent to prison over the last decade, according to an analysis of U.S. Sentencing Commission data.

The trend has divided lawmakers and officials in the courts and along the border. Some politicians believe the mass hearings should be expanded to deter illegal immigration. Others question whether the system actually affects people seeking to cross the border, while some contend the programs distract prosecutors from pursuing more serious crimes.

"There is a use of criminal justice resources that doesn't make sense. . . . Are we just running numbers so it appears we're doing more on immigration and drug offenses, or are we doing anything worthwhile?" said Chicago federal Judge Ruben Castillo, who retired in December after serving on the U.S. Sentencing Commission for 11 years.

Some of the expedited border court hearings are part of a program called Operation Streamline that began in 2005 in Del Rio, Texas, and soon spread to other Border Patrol sectors.

It is a departure from the previous "catch and release" policy and aims to stem the flow of illegal immigrants from Mexico. Before 2005, illegal crossers were sporadically charged with federal misdemeanors, but many were simply driven back across the border.

Operation Streamline and other fast-track programs speed illegal immigrants through accelerated legal proceedings, where most guilty pleas come in Spanish and thousands of Mexican citizens end up imprisoned each year for entering the country without papers.

The first time someone is caught entering the country illegally usually results in a misdemeanor that leads to deportation or a maximum of six months in federal custody. If they are caught again - as often happens - they are charged with a felony count called illegal reentry that carries at most a two-year prison sentence or more if they have a criminal history. Some felony charges ultimately are reduced to misdemeanors through plea bargains.

U.S. attorneys in numerous states, including the four flanking the southern border, have authorized accompanying fast-track programs for felony offenders that allow cases to be resolved rapidly.

The programs have raised concerns about defendants' constitutional rights and the sheer volume of work flooding the courts. Critics say the programs overburden the court system and distract authorities from prosecuting major crimes.

The U.S. Sentencing Commission tracks the number of people who repeatedly enter the country illegally in a broad sentencing category called immigration crimes.

The number of people sent to prison for the primary crime of unlawfully entering or remaining in the United States jumped from 6,513 in fiscal year 2000 to 19,910 in 2010. Many also were sentenced for that crime plus more serious offenses.