"A re-elect in 2020 would be very difficult for the President if amnesty goes with DACA and if a wall is not at least under robust construction by then," King said on CNN's "Erin Burnett OutFront," referring to a reported deal
on the Deferred Action for Childhood Arrivals program that Democrats signaled Trump made with them Wednesday.
King added he believed Trump's accomplishments in these areas could also make the Republican majorities in the House and Senate vulnerable to Democratic takeover.
The Iowa Republican was an enthusiastic Trump supporter during the general election campaign, and his comments on Thursday reflected dismay from some corners of Trump's base, who have taken his recent overtures to Democrats as a sign he may not enact his promises to institute a crackdown on undocumented immigrants and build a wall along the US-Mexico border.
King said he does not believe Trump has changed his mind on the wall, but acknowledged Trump had "telegraphed a diminishment of the resolve" he had during the campaign.
"I want to see that resolve restored," King said. "I want to go on record; I believe we get a wall built."
Trump dined Wednesday night with Senate Minority Leader Chuck Schumer and House Minority Leader Nancy Pelosi, the top Democrats on Capitol Hill, who said in a statement
that they had gotten a potential agreement on making permanent protections for people who had received assurances under the DACA program.
The Democrats said the agreement had been made "excluding the wall."
King said if Trump backed a plan that included amnesty for undocumented immigrants, it would undermine all campaign promises in the eyes of the public. He noted with some dismay Trump's shift from campaign rhetoric insisting the wall would be built starting on his first day in office to the continued delays from his administration.
"We were going to have the Rolls-Royce of a big, beautiful wall," King said, adding that Trump's more recent comments about immigration mean that "the Rolls-Royce is now going to be an overhauled jalopy."
King said he has seen his colleagues in Congress refer to the wall as actually a fence or surveillance measures instead of the physical barrier Trump promised, and such declarations ring hollow.
"The American people know what a wall is," King said.
King took issue with Trump's delayed decision on DACA, saying he expected Trump to end the program on day one and instead saw the program continue for months until the administration announced it would rescind it after six months in order for Congress to put in place a permanent fix.
King has spoken harshly about immigrants for years
, making a string of controversial remarks
about race, immigration and the effects of demographics on American culture.