(This chart, by the Migration Policy Institute, is from Think Progress.)
Donald Trump, and his Republican cohorts in Congress, continue to spew a torrent of vile rhetoric about undocumented immigrants. They want you to believe we are being flooded with these immigrants that are stealing good American jobs and boosting crime in this country.
It is not true. The truth is that undocumented immigration is currently at about a zero level (meaning as many are leaving the country as entering it). That "flood" just doesn't exist. And those immigrants who are here do not steal "good" American jobs. They take what they can find, which is the dirtiest and most dangerous and lowest-paid jobs -- the jobs American citizens don't want.
The Republicans know this. And they also know that the only effective way to curb undocumented immigration is to severely punish the employers that hire these immigrants. They won't do that though, because it would damage the flow of campaign funds from the business sector (who wants that flow of cheap labor to fill their dirty and dangerous jobs).
So, why all the vile rhetoric about "illegal" immigrants? Two reasons. First, it keeps their racist and xenophobic base happy. But most important, it paints all immigrants with a broad brush (both documented and undocumented). That helps them do what they really want -- to change immigration policy in general (documented immigration).
Why do they want to do that? The chart above explains it. Note that 60% of documented immigrants come from only 10 countries, and all of them are countries where the people are nonwhite. And a good portion of the other 40% are also from nonwhite countries. In other words, the current so-called "legal" immigrants are overwhelmingly nonwhite. Trump and other Republicans want to change that.
It is easy to understand why Trump wants the change. He is a racist, and always has been. His dream of "making America great again" is to maintain the white dominance and privilege in the United States -- which is being threatened by the shifting demographics.
It is also easy to understand why congressional Republicans want the change. They see the demographic change as threatening their political power. They know that the racist policies they support have driven nonwhite voters away from their party. Since they can't change those policies (something their base would not allow), they think they can do it by initiating an immigration policy that excludes nonwhites and encourages immigration of whites.
To accomplish this goal, they want to stop family-based immigration (which they label as "chain migration) and discontinue the "lottery" visa immigration (which helps immigrants from mainly nonwhite countries). They would replace this with what they call a "merit-based" system. That would give preference to immigrants who have education and skills (who they believe will be mostly white).
Their major campaign donors (corporations) would also love this, because it would allow them to pay lower wages to foreign workers (who will accept that to get into the U.S.). That would depress wages for educated and skilled American workers who would be competing with those foreign workers for the "good" jobs.
The desired changes to immigration policy by Trump and the Republicans would not be good for this country. That's why they are using the vile rhetoric against undocumented immigrants. They hope it will mask their real desire -- to institute a more racist immigration policy.