The caravan of people slowly making their way on blistered feet and thin hopes toward America’s southern border sometimes seems like an election gift to President Trump, giving him fresh meat to throw to his base on the eve of fateful midterm elections.The Central Americans, estimated at about 3,500 people, many of them women and children, have morphed in the president’s immigrant-bashing demagogy into an “onslaught of illegal aliens” concealing “criminals and unknown Middle Easterners,” all enabled by Democrats and, Mr. Trump “wouldn’t be surprised,” by George Soros, a favorite villain of far-right conspiracy-mongers.Mr. Trump is not sitting back and letting the barbarians in. He has ordered the Army — which is barred by law from performing police functions within the United States — to bolster the frontier, saying he will authorize soldiers to shoot if the trekkers start throwing rocks “viciously and violently.” “This is an invasion of our Country and our Military is waiting for you!” tweeted Mr. Trump on Monday.Mr. Trump, perhaps counseled by someone who understands the military’s longstanding rules of engagement, dialed back the threat a bit on Friday. “They won’t have to fire,” he told reporters. “What I don’t want is I don’t want these people throwing rocks.”Most of Mr. Trump’s description of the migrants is untrue or unwarranted. But none of it is surprising. Demonizing immigrants is his go-to move, from his “big, beautiful wall” to his call to end birthright citizenship. Not to mention the race-baiting campaign ad he tweeted featuring a Mexican immigrant who was convicted of killing two police officers. The United States has clear laws governing refugees and well-funded agencies to enforce those laws, and it’s an embarrassing waste of money to send troops to the border.The Democrats have come back with a resounding response. Resounding silence, that is, apart from a few potshots at deploying the Army, which Barack Obama, stumping in Florida, assailed as a “political stunt.” More typical was the retort of Nancy Pelosi, the House Democratic leader, when confronted by Mr. Trump’s talk of revoking birthright citizenship. “Clearly, Republicans will do absolutely anything to divert attention away from their votes to take away Americans’ health care,” she said.Clearly, Representative Pelosi was doing some diverting herself. No doubt health care is a more comfortable campaign issue for Democrats than the minefield of immigration policy, but the caravan is not simply a political sideshow concocted by Mr. Trump. Anyone who wants to defeat his bigoted politicking needs to do better than to try to change the subject. The right way to deal with the caravan crisis is to make clear that it is no crisis. The marchers pose no threat. The United States has clear laws governing refugees and well-funded agencies to enforce those laws, and it’s an embarrassing waste of money to send troops to the border. In fact, illegal border crossings have significantly declined in recent years. The country must and will continue to enforce the laws that control its borders, as Mr. Obama himself did as president during an earlier, actual surge of Central American migrants, when he took the difficult step of dispatching National Guard troops to the border and detaining many mothers and children. Longer-term questions about how to put the country’s approach to immigration back on a rational, moral foundation are more difficult. Republican hard-liners defeated bipartisan attempts at comprehensive immigration reform in 2007 and 2014. In the Trump era, Democrats have found the issue of immigration even more confusing; a couple of Democratic senatorial candidates have even lined up behind Mr. Trump.Mr. Trump’s cruel treatment of immigrants and race-baiting about nonexistent threats do not amount to a solution. Managing the entry of refugees and other immigrants, and creating a fair system to deal with the millions of undocumented immigrants within the borders, are serious matters in need of common sense and elemental humanity. The country needs to streamline the asylum system and establish generous quotas of immigrants and refugees from around the world. To be effective, any immigration plan has to include serious development aid to Central America’s troubled states. Cutting off what little aid they get, as Mr. Trump has threatened to do, will only create more caravans. People seeking to partake of the American dream have always been central to America’s identity and strength. How the country treats them goes straight to its core values. The Democrats cannot sit this one out, especially when the Republican leader is so blind to the true sources of America’s greatness.Follow The New York Times Opinion section on Facebook, Twitter (@NYTopinion) and Instagram.