The ZPG, the trade Union and Treehugger factions seem hold sway among the liberals, I only hear open borders arguments from conservatives. I mentioned George Guilder earlier. It's hard to question HIS conservative credentials. He advocated essentially open borders in his book "Wealth and Poverty" which was the conservative manifesto of the Neo-Conservative revolution.
Historically, each wave of immigration was a net gain for the US. Each wave was also opposed by those already here. "NO IRISH NEED APPLY"! for instance.
What makes ths country great ultimately is virtue. It isn't abstract morality if history shows the beneficial result from allowing others the same freedoms and opportunities as our forefathers enjoyed when they arrived.
We need to fight liberalism directly, on the battlefield of ideas. Political intrigues like worrying about who gains an advantage due to immigration are an empty exercise if we don't possess a superior ideology and the powers of persuasion enough to win over the hearts and minds of men.
Rather than fight immigration and post armed guards at the borders we need to prevent the indoctrination of people everywhere with the falsehoods we oppose. If we can't do that then we will never be able to build walls high enough to keep out the Hun for they are already amongst us.