WASHINGTON — Democratic presidential contenders are in a feverish battle to one-up each other with ever-more-ambitious plans to beat back global warming , curb gun violence , offer universal health care coverage , slash student debt and preserve abortion rights . Largely left out of the policy parade: immigration.

The field of 24 candidates is united in condemning President Donald Trump’s support for hard-line immigration tactics, particularly his push to wall off as much of the U.S. border with Mexico as possible, roll back asylum rights for refugees and since-suspended efforts to separate immigrant children from their parents. But only two contenders — ex-Obama Housing Secretary Julian Castro and former Rep. Beto O’Rourke — have released detailed, written policies addressing the future of the immigration system.

The dearth of formal policy plans signals the challenge that immigration could pose for Democrats. White House hopefuls can easily rally their party’s base with broad, passionate attacks on what they see as Trump’s failures, but it’s riskier to grapple with the complexity of the immigration system. Trump, meanwhile, has tapped into fervor around immigration to energize his own supporters and has worked to seize on it as an issue of strength — territory Democrats risk ceding to him ahead of 2020 if they don’t find a way to go deeper.

“For the most part, the Democrats aren’t even trying to make the case to a centrist voter of what a reasonable immigration plan would look like,” said Ali Noorani, executive director of the Washington-based National Immigration Forum, which works with faith leaders and law enforcement to promote the value of immigration. Undecided voters “know that Trump’s simplistic approach to this isn’t working,” Noorani said, “but they’ve got nowhere else to go.”

The issue isn’t likely to recede as the presidential campaign intensifies. Much of the Democratic field is heading this weekend to California — it borders Mexico and is home to the largest Hispanic population in the U.S. — for a state party convention. Meanwhile, the U.S. Border Patrol has said it plans to fly hundreds of immigrant families out of Texas as it struggles to process the large numbers of Central American families that are reaching the U.S. border with Mexico and asking for asylum.