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Immigration has always been President Trump's signature issue, from the day he rode down the golden escalator at Trump Tower and talked about Mexican rapists to his warning about approaching caravans of migrants during the midterm campaign. Most importantly, for the president, he sees it as the issue that motivates his core support. NPR's Mara Liasson has this report.
MARA LIASSON, BYLINE: Donald Trump always trusts his gut, and he believes he has a keen understanding of the political perils for him in this latest debate about immigration. In the Oval Office yesterday, he spelled them out.
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PRESIDENT DONALD TRUMP: If I did something that was foolish, like gave up on border security, the first ones that would hit me are my senators. They'd be angry at me. The second Hmongs would be the House. And the third ones would be, frankly, my base...
LIASSON: His base, as he often points out, is behind him all the way. And he does have very strong approval ratings among Republicans. Matt Schlapp, chairman of the American Conservative Union, says a big reason for that support is immigration.
MATT SCHLAPP: The base of the party actually wants to see security first. And they want to see that southern border sealed. And so Donald Trump is the candidate, and now the president, who identifies with that very strong conviction for Republicans and for conservatives. He has a great deal of credibility on the issue of immigration with these folks.
LIASSON: A great deal of credibility, but apparently not unlimited credibility. In private, several of the president's advisers state flatly that Trump can't win re-election if he doesn't build the wall or at least do everything in his power to get it done. Back in December, when the president indicated he might sign a budget bill without wall funding, he got tremendous pushback from conservative talk show hosts, who Trump's advisers believe speak for a lot of Trump supporters.
It turns out his base took him literally and seriously when it came to the wall. Fox News host Laura Ingraham reacted against Trump's assertion that the wall was already being built. In her view, merely renovating current border barriers just wouldn't cut it.
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LAURA INGRAHAM: That's not a wall. Stop saying it's a wall. There's no wall. If you want a wall, say we don't have the wall and - I know it's bad because he made the promise, but they're not building the wall.
LIASSON: But now, President Trump has come up with a way to keep faith with his base even if he can't convince Democrats to fund a wall. If a compromise is impossible, the president said today, he'd, quote, "definitely" do an end run around Congress, declare a national emergency and build the wall with unobligated Pentagon funds. And even if that gambit is stopped in court, he'll have shown his supporters he did everything he could. Pollster John McLaughlin, who worked for Trump during the 2016 campaign, says in that case, his base will be satisfied.
JOHN MCLAUGHLIN: They see him fighting. And if they realize the Democrats don't give it to him, they've seen him fight the good fight.
LIASSON: Polling on immigration depends a lot on how the questions are worded, but almost every poll shows that while border security is broadly popular with voters, the wall and shutting down the government to get a wall is not - except among Trump's core supporters. And now that Trump is promising to act alone if necessary, he's getting praise from some of the same conservatives that previously attacked him for waffling on the wall. Over the weekend, Rush Limbaugh told his listeners that Trump has to do this all by himself.
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RUSH LIMBAUGH: It remains a one-man show, Donald Trump against the Democrat Party and the media. We are very fortunate the guy does not cave.
LIASSON: Regardless of how the shutdown and the battle over the border is resolved, this fight will not end in 2019. Today, on his way out of the White House, Trump made sure to describe the border issue as more than just a wall. The Democrats, he said, don't care about crime.
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TRUMP: I really don't think they care about crime. And, you know, sadly, they're viewing this as the beginning of the 2020 presidential race. And that's OK with me.
LIASSON: It's OK with him because he sees it that way too, as he tweeted in December, quote, "we have the issue - border security 2020 - exclamation point." Mara Liasson, NPR News, Washington.