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 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.