WASHINGTON — As Republicans try to keep their midterm election strategy focused on the economy, tax cuts and falling unemployment, President Trump sent his clearest signal yet on Monday that he intends to make divisive, racially charged issues like immigration central going into the campaign season.

Facing bipartisan criticism over his administration’s family separation practice on the border, Mr. Trump renewed the sort of bald and demagogic attacks on undocumented immigrants that worked well for him politically in his 2016 presidential campaign. He inveighed against “the death and destruction that’s been caused by people coming into this country” and vowed that “the United States will not be a migrant camp and it will not be a refugee holding facility.”

Republicans typically handle immigration gingerly in an election year, as they try to appeal to Hispanic voters, independents and moderates across divergent districts. But with more Americans still opposing the tax measure than supporting it, Mr. Trump’s allies believe that trying to link Democrats to crimes committed by undocumented immigrants and gangs like MS-13 will do more to galvanize Republican voters and get them to the polls in November than emphasizing economic issues.

“People don’t turn out to say thank you,” said Corey Lewandowski, one of the president’s top political advisers. “If you want to get people motivated, you’ve got to give them a reason to vote. Saying ‘build the wall and stop illegals from coming in and killing American citizens’ gives them an important issue.”