Plain and simple.
Free movement between states.
@Ryan Israel People can now freely enter the United States, they can use the resources you and I pay for. Many of them bring danger, like the MS-13. The reality is, without screening who comes in to the United States, is terrifying. Most drugs come internationally, and that on its own is a huge issue. People flee their countries because of different governing systems, whereas different states in the US have the same. We cannot treat people coming from other countries like we treat people from other states.
@Ryan Israel When you open your arms to everyone in the world. Terrorism rises, economy slows down, government has to take care of them. I ask you, do you support handing anyone who crosses the border citizenship?
@Chris Johnson Legalize drugs so that people don't have to go through gangs and cartels to get them so that it makes those gangs and cartels less powerful. Also, free movement doesn't mean they're a citizen the moment they enter the country, you would still need to be a citizen in order to do things like receive welfare. Your claim about immigration slowing down the economy is blatantly false. https://www.unidosus.org/issues/immigration/resources/facts
Jobs poorer and undocumented immigrants are likely to take are low-skill manual labor jobs that many people in the United States aren't taking because they view these jobs as beneath them.
As for your claim about rising immigration rates bringing higher rates of terrorism, you are right. A study performed by Robert J McAlexander found that an increase of immigration does lead to an increase in terrorism, except the terrorism isn't being committed BY the immigrants but rather TOWARD the immigrants.
So you want a border that has zero restriction, no requirement of identification, and no border control? Or do you mean a conditionally open border, which would make more sense?