For those who enter the US illegally across the Mexico–United States border and elsewhere, migration is difficult, expensive and dangerous. Virtually all undocumented immigrants have no avenues for legal entry to the United States due to the restrictive legal limits on green cards, and lack of immigrant visas for low-skilled workers. Participants in debates on immigration in the early twenty-first century called for increasing enforcement of existing laws governing illegal immigration to the United States, building a barrier along some or all of the Mexico-U.S. border, or creating a new guest worker program. Through much of 2006 the country and Congress was immersed in a debate about these proposals. few of these proposals had become law, though a partial border fence had been approved and subsequently canceled.