Even with changes, U.S. prisons might stay overcrowded: report WASHINGTON (Reuters) - Overcrowding in the U.S. federal prison system is so severe that the problem could go on for years even if Congress takes some steps to reduce the number of people behind bars, according to a report to be released on Tuesday. On the other hand, the report from the nonprofit Urban Institute said that lawmakers have numerous options if they want to start making some dents in a prison population that is the largest in the world. The United States incarcerated 2.2 million people in state and federal institutions in 2011, the most recent year for which the U.S. Bureau of Justice Statistics has published data. The huge size is largely the result of mandatory prison terms enacted in the 1980s and 1990s while crime was on the rise. Those policies might now be falling out of favor because the expense and the social effects of locking up people who are disproportionately members of racial minorities. Unusual pairings of liberal lawmakers and Tea Party conservatives are pushing legislation to reduce federal prison terms or give judges more flexibility in sentencing. The report from the Urban Institute, a research group with roots in President Lyndon Johnson's Great Society anti-poverty program of the 1960s, shows how difficult it would be to bring the prison population in line with capacity. Even if Congress were to reduce mandatory minimum sentences for drug crimes by half, an idea with dubious political prospects, federal prisons would still be 20 percent above capacity in 10 years, the report said. They would be 55 percent above capacity if policies went unchanged, it said. "As much as there are many good policy ideas out there, it's going to take several of them to even get to the point where prisons are not overcrowded," said Nancy La Vigne, director of the Urban Institute's Justice Policy Center. The report also details the potential budget savings from a menu of changes under discussion in Congress. If lawmakers were to apply retroactively new prison terms that they approved in 2010 for crack cocaine-related crimes, they would cut prison spending by $229 million over 10 years and free up 22,000 "bed-years." One person released 12 months early frees up one bed-year. La Vigne is one of six witnesses invited to testify about a prison overhaul on Wednesday before the U.S. Senate Judiciary Committee. Others scheduled to appear include local officials from Kentucky and Pennsylvania and Charles Samuels, director of the Federal Bureau of Prisons. The issue is a top priority for U.S. Attorney General Eric Holder, who on Tuesday is scheduled to visit a Philadelphia program designed to reduce recidivism. "There's been a tendency in the past to mete out sentences that frankly are excessive," Holder said at a news conference on Monday. Given financial constraints, he said, "we have to really rethink our priorities." Editing by Howard Goller and Andre Grenon