Here is just one of the reasons why the open borders became so important to the democrats after Trump was elected. Young black Americans took a stand to break the chains of slavery by leaving the democrat party by the thousands. And along with them, they also captured the attention of thousands more.
Not only did the democrats lose thousands and thousands of their black voter base, they also lost thousands from their white voter base. it became urgent that they replace those voters. And since some states were cracking down on voter fraud so heavily, it became necessary that they get millions of illegals in the country, cater to them and allow them to vote!
That is it in a nutshell, the democrats need the border open so their stream of voters has access to this country. They could care less if the drugs, diseases, criminals, etc. came with them as long as them got the votes.

The Young Black Conservatives of Trump's America

27 January at 13:00 ·

Donald Trump received more votes from black voters than both Mitt Romney and John McCain.

VICE met with young black conservatives around America to hear about what’s drawing them to the Republican party, along with what kind of backlash they’ve faced for going public with their political views.