‘Dangerous people are coming here and the good people are dying,’ Trump Warns in Texas Visit.

SAN ANTONIO, Tex. — President Trump said on Wednesday that migrants pouring across the border with Mexico are dying in great numbers while other gang members arriving from Central America are marauding and threatening American ranchers.

The president used a high-dollar fund-raiser here to call attention to a situation that he said has been ignored in the media: the plight of migrants who cross illegally into the United States and then die of thirst or hunger.

“This never comes out in the fake news,” Mr. Trump said as he recounted the stories about migrants that about a dozen donors told him privately at his first stop in a visit to Texas that will take him to Houston later in the day. At Mr. Trump’s urging, several of the donors described finding the bodies of migrants — including pregnant women and children — in the vast brush of their property.

The president said that he had never heard the stories of migrants dying, even from his top immigration and border patrol officials. In fact, migrant advocates have for years documented the grim fate of some migrants who grow sick and die attempting to make it into the United States. The advocates say Mr. Trump’s policies have made the problem worse by limiting the number of migrants who can legally claim asylum at ports of entry, pushing more migrants to cross at remote areas of the border.

Several of the donors also told of how afraid they have felt when migrants from Central America, dressed in black, turned up at their homes.

“Dangerous people are coming here and the good people are dying,” Mr. Trump said, adding that the donors had all told him that the answer to the problem was to build his wall along the border with Mexico.

The president, who was joined at the round table with donors by Brad Parscale, his 2020 campaign manager, denied that the unscheduled remarks to reporters about the border were part of a campaign message. But moments later, as he attacked Democrats for failing to address border security, Mr. Trump said that immigration would be a tremendous issue for him and other Republicans in the 2020 campaign.

“I think they are going to pay a very big price in 2020,” Mr. Trump said. “I think the border is going to be an incredible issue. They want to have open borders.”

The issue of immigration and border security has been at the center of Mr. Trump’s political life for years. Fifteen days before the 2018 midterm elections, he held a rally in Texas to deliver dire warnings about immigration that helped Ted Cruz, the embattled Republican senator, win his campaign for a second term.

In the five months since he barnstormed the country declaring that an “invasion” of dangerous migrants was imminent, Mr. Trump has intensified his focus on immigration. He and his strategists believe that no issue better fires up his core supporters and proves that he has kept his campaign promises. The issue is certain to be at the center of the president’s case for a second term in the Oval Office.

In the last several days, Mr. Trump has forced out Kirstjen Nielsen, his Homeland Security secretary, and several other top immigration officials for being too timid about shutting down the border and changing asylum rules to deny entry to migrants seeking protection in the United States. A top administration official said Tuesday that the staffing changes were designed to make way for more aggressive immigration actions.

As he travels in Texas on Wednesday for a series of fund-raisers and round tables with supporters, Mr. Trump will not be alone in discussing the importance of immigration and America’s role in confronting the plight of displaced people.

Julián Castro, the former housing secretary under President Barack Obama and now a Democratic presidential candidate, will host a rally in San Antonio, his hometown, that will focus on immigration, setting up a split screen in this border state that underscores the issue’s potency for 2020. Mr. Castro’s rally is expected to feature English and Spanish speakers and is being billed by his campaign as an opportunity for San Antonio, with its significant, longstanding Mexican-American population, to show its resistance to Mr. Trump and his border ideology.

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It is not clear whether the president will acknowledge Mr. Castro or the Democratic rally, which is scheduled to take place after Mr. Trump has already traveled from San Antonio to Houston Wednesday evening. The president has sometimes chosen to ignore critiques from candidates he deems not to be a political threat.

But if he does talk about that rally, Mr. Trump could provide an early example of how the immigration debate will play out on the 2020 campaign trail.

Mr. Trump is betting that the dark and threatening rhetoric — which worked in 2016 and appeared to work in some places, like Texas, in 2018 — will be just as effective in his re-election campaign. In a speech to the Conservative Political Action Conference in March, Mr. Trump accused Democrats of abandoning the mainstream on immigration, adding: “But that’s going to be good for us in 2020. They’re embracing open borders.”

But Mr. Trump does face challenges of his own. His promises on immigration from the 2016 campaign have mostly gone unfulfilled. He has largely failed to build the “big beautiful wall” along the southern border as he promised. And the recent surge of migrant families from Central America is a vivid demonstration of his inability to stop what he has called an “invasion.”

There was also significant evidence during the 2018 midterm elections that the president’s immigration attacks backfired in some Republican districts around the country. For example, several House Republicans, including some of the party’s leadership in Congress, complained to Mr. Trump that his announcement right before the election that he was considering an executive order to end birthright citizenship may have cost several moderate Republicans their seats.

And as he sets out for the re-election campaign, Mr. Trump is certain to face several challengers among the Democrats who are determined to make the president’s immigration agenda a key part of their reason for running.

Mr. Castro, the only Latino in the ever-expanding Democratic presidential race and the grandson of a Mexican immigrant, has seized on immigration as a central issue in his fledgling campaign. Last week, he became the first candidate to unveil a detailed immigration plan that aims to roll back the Trump administration’s hard-line immigration policies. Included in Mr. Castro’s immigration platform are proposals to decriminalize unauthorized border crossings; provide a pathway to citizenship for undocumented immigrants; and split the Immigration and Customs Enforcement agency in half and reassign enforcement duties to other agencies.

Under his policy, Mr. Castro also called to establish a so-called Marshall Plan for Central America to aid countries that have a high number of migrants, including by increasing funding for economic development and violence-prevention programs.

With his immigration proposals, Mr. Castro, who has also served as mayor of San Antonio, is trying to position himself in the race for his party’s nomination as the candidate who can best combat Mr. Trump’s contentious border policies.

Supporters of Mr. Castro say his background, including his deep ties to this border state — his mother, Rosie Castro, is a civil rights activist who was among the leaders of the La Raza Unida political party — position him to address immigration issues. Some Democratic strategists also view his presence in the race, along with that of the former Texas congressman Beto O’Rourke, as instrumental not just in pushing immigration to the fore but also in mobilizing Latino voters in Texas and elsewhere.

“Texas is a Latino state, and we have two Texans on the ballot who are going to be turning out and galvanizing people from Texas,” said Mayra Macias, vice president of the Latino Victory Project. “But it’s also helping galvanize people, Latinos in particular, across the country because they are seeing these candidates talk about issues that affect us.”

The dual candidacies of Mr. Castro and Mr. O’Rourke are almost certain to place Texas squarely at the center of the increasingly heated immigration debate. But if immigration is at once a key campaign issue in Texas, and other states including California and Arizona, Republicans are betting that Mr. Trump’s anti-immigrant message will also resonate far from the southern border.

It was only two months ago that Mr. O’Rourke provided his own counterpoint to Mr. Trump’s border exhortations, with a rally in the border city of El Paso that coincided with one held by the president. “We are not safe because of walls but in spite of walls,” Mr. O’Rourke told supporters even as Mr. Trump was pressing to “finish the wall.”