Even the most partisan Democrats are hard-pressed at this point to deny that the border crisis President Trump has long been warning about actually exists.

Yet that hasn’t stopped them from reflexively denouncing pretty much any action he takes — without suggesting any solution or ideas of their own.

Trump is threatening to close all or part of the US border with Mexico, mainly the 50 legal crossing points. (It’s been done before, by three other presidents.)

This after the White House announced plans to cut off some $500 million in aid to the Northern Triangle countries — Guatemala, Honduras and El Salvador — that are the major source of what officials call an “unsustainable” flood of asylum seekers. The president is also looking at naming an “immigration czar” to manage the crisis.

More than 76,000 crossed the border in February; the March total is expected to top 100,000.

Most of these are families — or, at least, claiming to be families. That’s because smugglers, who get paid thousands of dollars, pair adults with unrelated children to exploit a fast track to sanctuary.

US immigration and asylum laws written decades ago desperately need updating, especially as courts have interpreted them to make the border even more porous and handcuff attempts to control it.

As things stand, migrants need no longer claim a “well-founded” fear of persecution (as the law requires) if they’re returned home. The Trump administration’s attempt to impose conditions for asylum was overturned by the 9th Circuit Court of Appeals.

Court rulings also prevent detention of migrants for more than 20 days; most are simply released and allowed to stay in the US while they wait for immigration hearings — assuming they show up.

The main barrier to changing any immigration laws has been the tendency of both sides to insist on reforming everything at once. That’s plainly the wrong way to go here: What’s needed are changes that don’t effectively grant a right to stay in the United States to anyone who says the right words.

Democrats wax indignant whenever the president threatens to take action, but don’t offer anything more realistic than “Let them all in.”

Trump recognizes the crisis. Democrats need to get out of the way.