Neither Republicans would really want all out, nor Democrats would really want all in. The result of the duopoly system is a constant, controlled influx of immigrants. All they do is fight publicly in the media about more or less influx, it results in a draw.

Once the immigrants arrive they are exploited for cheap labour, some of their children being high achievers that feed the elite academic institutions (and "the best" the government doesnt even need to pay for their tuition fees as would be the case in many other Western countries).

The overall system is evil and designed for the economy. Also note that from a longterm perspective, its probably disadvantageous for those poor countries to have steady outflux of their assumably upper percentile of intelligent people. If one let people in by merit (work skills or academic achievements) is even more dehumanizing - something they are thinkinh about for Chinese immigrants, to steal the top talent there.

In Europe immigration is killing thousands every year through the dangerous routes in the Mediterranean Sea.

When Trump wanted to build a wall, the media let it seem like a crackpot idea: "We build bridges, not walls", "Wall will not be effective", "Its childish to think a wall can help, there are water ways and the border is much too long", etc. Then when Finnland considered building a wall to block Russian men from enterying the country, it was approved by the media as something to consider, and the wall as something effective.