My defense of open immigration is mostly an attack against conservatives. But conservatives are not uniformly opposed to a more humane, just, and open immigration policy. An obvious example is the most recent conservative president – George W. Bush. I dislike George W. Bush immensely, partly for his initiative in expanding the welfare state. But I believe that he sincerely wanted to improve immigration laws by creating a guest worker program and a pathway to citizenship for illegal immigrants (i.e. some sort of amnesty). John McCain, hate him or hate him, also worked hard to keep the failed 2006-7 immigration reform push alive. Ronald Reagan famously signed a bill granting amnesty to illegal immigrants.
I suspect that the conservative elite is ahead of its base on immigration. Maybe elites value economic thinking more and support labor mobility. Maybe the elites have broader horizons – they are more cosmopolitan and have more contact with foreigners, so they are less able to ignore their humanity. Maybe conservative politicians are just worried about the future of the conservative party, given a shrinking Caucasian share of the electorate.
Their secret convictions notwithstanding, conservative politicians have failed to make progress in the last few decades. Politicians are professionals at giving voters what they want – so the blame ultimately falls on the conservative base that the politicians are afraid to challenge. Until the conservative base learns to see immigrants as equal human beings, the conservative party will continue its pointless war against foreigners.