Several family members at the news conference echoed the president’s phrasing about their permanent separation from their loved ones. The sound bites aired repeatedly over the following days in the conservative media.

Their advocacy has not come without costs. The families have been accused of using their children’s deaths to provide cover to Mr. Trump and his immigration crackdown, including in letters and postcards mailed to their homes after their appearance with the president last month.

“Sadly, you have now despoiled your loss, your own good name, and the name of your loved one to serve the political ambitions of an evil, soulless man,” read an unsigned letter mailed last month to the homes of at least two of the mothers, copies of which were provided to The New York Times. A father who spoke at the White House received an unsigned post card afterward comparing him to “the loyal followers of Adolph Hitler in 1934,” misspelling Hitler’s given name.

Nonetheless, several of the parents said in interviews that they stand ready to champion Mr. Trump’s re-election campaign, and they are not waiting for 2020 to use their newfound political clout. They are helping to raise money and build support for advocacy groups of their own, as well as for 2018 campaigns focused on limiting immigration, including that of Joe Arpaio, the former Arizona sheriff who declared his candidacy for Senate only months after Mr. Trump pardoned his conviction for defying a federal judge’s order to stop targeting Latinos based solely on suspicion of their immigration status.

“The hell with political correctness — we need new blood. And you bet I’m going to be out there campaigning for people who are going to do something to make our country safer and secure our borders,” Mary Ann Mendoza, one of the mothers who spoke at the news conference, said in an interview. Her son Brandon Mendoza, a Mesa, Ariz., police officer, was killed in 2014 in a head-on collision with a car driven by an intoxicated undocumented immigrant.

She and other similarly affected family members first shared their stories with Mr. Trump soon after he started his 2016 presidential campaign, and he brought them onstage with him to repeat their stories at key points throughout the race.