Forty percent of illegal aliens are not border crossers, but people who came on legal visas and then remained in the US. in violation our law. Congress authorized an effective tracking system in 2001 to determine whether visa holders leave, but it is still not operational. Sen. Marco Rubio (R-FL) supports amnesty, but says it should not happen until a tracking system is in place. The Obama Administration has not shown particular interest in tightening the tracking system.