President Obama answers questions during his news conference at the White House on Tuesday. Pablo Martinez Monsivais/AP hide caption

itoggle caption Pablo Martinez Monsivais/AP

American farms like this iceberg lettuce field owned by Duda Farm Fresh Foods outside Salinas, Calif., are facing a dwindling supply of farmworkers from rural Mexico. Kirk Siegler hide caption

itoggle caption Kirk Siegler

President Obama during his news conference Tuesday at the White House. Larry Downing /Reuters /Landov hide caption

itoggle caption Larry Downing /Reuters /Landov

ATF agents search last week for evidence at the site of the fire and explosion in West, Texas. Tom Reel/The San Antonio Express-News/AP hide caption

itoggle caption Tom Reel/The San Antonio Express-News/AP

Democrat Elizabeth Colbert Busch and former South Carolina Gov. Mark Sanford debate for the South Carolina 1st Congresional special election in Charleston, S.C., on Monday. Randall Hill/Reuters /Landov hide caption

itoggle caption Randall Hill/Reuters /Landov