The economy has seen an "unprecedented" number of long-term unemployed under the Obama administration, according to a liberal think tank, and economists say plans pursued by Democrats in Washington are unlikely to curb the problem.

Nearly 5 million workers are classified as long-term unemployed, while 900,000 more have stopped looking for work altogether, according to a new series of reports issued by the Urban Institute.

Three percent of the labor force has been out of work for more than six months, an improvement of only one percentage point since unemployment spiked in October 2009, according to the study.

"That long-term unemployment would rise during a recession is not at all surprising, but the extent of the increase and its persistently high level since the start of the recovery are both troubling and unprecedented," the report states. "The U.S. economy is now well into its fourth year of recovery, the unemployment rate is below 8 percent, yet the long-term share of unemployment is still near 40 percent."

The center-left think tank said that those startling figures are unlikely to change unless the United States can achieve dramatic job growth, rather than the middling 2 percent overall economic growth figures the Obama administration has averaged.