Securing America’s porous Southern border was Donald Trump’s signature issue when he was running for the White House two years ago. His “Build the Wall” chants, however disconnected they were from policy reality in Washington, galvanised angry voters and allowed Trump to steamroller his GOP rivals and then Hillary Clinton – all the way to Pennsylvania Avenue.Now, 18 months into his presidency, the Wall remains as imaginary as ever, but Trump’s core issue stands tall, as emotive and effective for him as it ever was. For all his political shortcomings, Trump retains the priceless advantage of possessing highly cooperative adversaries who, through fanciful indiscipline, keep turning debates around to the president’s benefit.Take last month’s “kids in cages” debacle, which placed Trump in the awful position of defending a family separation policy among asylum seekers appearing on America’s borderNo matter how many caveats were proffered by the administration – the images weren’t all that bad, the media was embellishing the story, plus Obama did the same thing – the political message was devastating. For a few days. Then, in deus ex machina fashion, the left-wing of the Democrats saved Trump from himself yet again.In a minor primary race for a safely Democratic House seat in New York City, the 28-year-old political ingénue Alexandria Ocasio-Cortez ousted Joe Crowley, a top Democrat in the House hierarchy, by running as a media-savvy left-wing Latina millennial, in a district that during Crowley’s two decades in Congress had demographically shifted to become half-Hispanic. Ocasio-Cortez’s platform was the customary Sandersian jumble of free-stuff-for-everyone, combined with a push to abolish Immigration and Customs Enforcement. ICE, which is part of the Department of Homeland Security (and like DHS was created after 9/11 in a security-driven shakeup of the Federal government), enforces immigration laws, as its name states, and therefore has become the Left’s hate object du jour. Chants of “Gestapo” have become rote among the ICE-haters, while Ocasio-Cortez’s primary victory launched #AbolishICE from a Twitter hashtag pushed by the self-proclaimed “Dirtbag Left” into the Democratic mainstream.Overnight, leading Democratic politicians, including high-profile Senators Kirsten Gillibrand of New York and Elizabeth Warren of Massachusetts, discovered that they, too, want to disband the nation’s immigration enforcement agency. Since nobody can explain what #AbolishICE would mean in practical policy terms, it seems safe to assume that this is simply a call for open borders without naming it such. Thus, as if by magic, did a genuinely radical position once favoured only by hardcore libertarians enter the mainstream of the Democratic Party.It didn’t take long for Democratic leading lights to realise that such a sweeping idea spells death at the ballot-box. Opinion polls are admirably clear that few Americans want open borders. Last weekend, Jeh Johnson, who headed DHS in Obama’s second term, took to the op-ed pages of The Washington Post to warn his fellow Democrats of the fire they have decided to play with. ICE does plenty of good and necessary work to protect Americans, Johnson explained, memorably adding, “The reality is that abolishing ICE is not a serious policy proposal; it’s about as serious as the claim that Mexico’s ‘gonna pay for the Wall’.”Already top Democrats are attempting to back away from #AbolishICE, without offending the ardent activists whose votes they will need this November in the midterm elections whichstand as a de facto referendum on President Trump – and may decide his fate in future impeachment hearings once Robert Mueller’s investigation wraps up. That is a tall order, however, and much damage has already been done. True to form, Trump has taken to Twitter to castigate the Democrats for their newfound support for open borders.For all his online bluster and disingenuousness, Trump is on rather firm ground here: Democrats have shifted, while he has not. That the Left has abandoned Obama-era centrist positions on immigration – enforcing Federal law with compassion, deporting serious criminals rather than run-of-the-mill illegal immigrants – is openly admitted by the Left itself, with consequences that may prove dire come November. Simply put, if the choice is binary, between a left-wing appeal to open borders and a Trumpist call for rigorous border enforcement, there is no reason to doubt that significantly more Americans will chose the latter over the former. Moreover, in his turbulent fashion, Trump has detected a serious weakness in the Democratic position, which conflates immigration with unchecked migration. Most Americans support the former but not the latter.While Trump overstates when he terms America’s immigration laws “the weakest and worst anywhere in the world,” he hits on the fact that, for many decades, the country has had a strange, two-tier system. The official immigration system privileges family unity – rather than the skills of immigrants as in Canada or Australia – and is cumbersome and expensive to use. Yet the unofficial system skips all that, allowing the entry of millions of migrants in a deal that helps employers to obtain low-skill (and low-cost, because illegal) labour while burdening local schools and social services. Most Americans know that our current incoherent system is untenable and needs root-and-branch reform, which seems out of reach in a divided Washington. Now the issue of mass migration looms, as it does across the Western world. The increasingly violent chaos of Central America means more migrants will be coming north, while the recent election of the left-wing nationalist Andrés Manuel López Obrador as Mexico’s president promises nothing resembling calm. AMLO, as everybody knows him, has some Trumpian traits himself, and seems bent on confrontation with America’s openly anti-Mexican president – a joust which The Donald will surely relish – while his stated position that Latin Americans have a “human right” to migrate to the United States seems sure to deliver fireworks.As tumult on our southern border rises, and America’s mass migration problems increasingly resemble those of the European Union and Australia, Trump stands to benefit. We can expect endless “I told you so” tweets from thWhite House. However, until the Democrats can re-plot a course to a centrist position which defends American borders while treating immigrants and migrants humanely, the president will keep winning on his signature issue.