Immigration — the issue that helped propel Donald Trump to the presidency and rally his supporters over the past decade — has become an afterthought for voters.

Why it matters: It's another ominous sign for Trump and the GOP heading toward November. What was once a rallying cry for the party is losing salience with voters across the spectrum — and harsh crackdowns have even made it a liability among some Republicans.

  • "On net, Republicans win out as the more trusted party on only two topics, and it's border/immigration [and] crime and safety," said Avery James of Echelon Insights, a Republican polling firm. "But the problem if you're a campaign strategist is quite simple: crime and immigration are just not the top issues for voters."

The big picture: Just two years ago, immigration was a top concern for voters nationwide, especially self-identified Republicans.

  • After President Biden dropped his reelection bid, the GOP spent half its advertising budget on immigration-related ads, per an AdImpact analysis.
  • "If you ask [about] immigration as an overall issue that people care about, it went from like a top three issue two years ago to way down the list now, in single digits," said Erik Iverson, a GOP pollster working with the Republican Senate campaign arm.

Between the lines: Pollsters say concern about immigration peaked during Biden's tenure, when illegal border crossings and asylum seekers at the Southern border surged.

  • But weeks into Trump's term, crossings dropped dramatically after he deployed the U.S. military at the border, instituted a ban on asylum claims and paused refugee admissions.
  • "You become a victim of your own success," Iverson said.

Zoom in: Cost of living ranks as the top issue for voters, far outpacing immigration. That's especially bad news for Republicans trying to turn out voters in competitive statewide races.

  • More than half of Texas Republicans and one-quarter of independents said border security and immigration was Texas's most important problem about one year ago, according University of Texas poll. Its latest survey, conducted in August, shows that figure cut in half.
  • In Michigan, only 14% of all voters said it was their top issue in a survey for AARP co-conducted by Trump pollster Fabrizio Ward.
  • A September poll of Ohio voters shows a higher share concerned about immigration, 25%, when asked to name their top three issues. But it was dwarfed by a combined 70% who were most concerned about the economy and inflation.
  • "That issue just dropped with Republicans because they weren't seeing it every day. They weren't feeling threatened by it every day," said Brent Buchanan, of Cygnal, a Republican polling firm.

The intrigue: Tying immigration to the economy and cost of living still resonates with low-propensity Trump voters, according to polling Iverson recently shared with Senate Republicans.

  • And Buchanan said that border security can still be used effectively to draw a contrast between candidates in specific races. But it's unlikely to drive turnout among what he called "soft Republicans."

By the numbers: Voters are starting to sour on Trump's handling of immigration.

  • His disapproval rating on immigration jumped to 58% in a late August survey from Ipsos, up from 38% when Trump took office last year.

Stunning stat: A September New York Times/Siena poll showed Democrats are more trusted on immigration than Republicans by 51 to 45%. Most other polling, however, still shows Republicans ahead on the issue.

  • "Everyone likes enforcement until it hits close to home or they get the wrong person. It's not necessarily the policies. It's the tactics that are turning people off. There has to be a middle ground," a former White House official said.

The other side: "Americans are less concerned about immigration because President Trump stopped the invasion of our country and delivered the most secure border in American history," White House spokeswoman Lauren Bis said in a statement. "Thanks to President Trump's strong immigration enforcement policies, approximately three million illegals have left the United States and zero illegals have been released at the border in sixteen months."

What they're saying: The polling trends are giving Democrats a new opening, especially in areas with sizable Hispanic populations.

  • Republicans are "trying so hard to distance themselves from Trump on the overall immigration issue because they know it is toxic with Latino voters right now," said Stephanie Valencia, founder of the Latino-focused polling firm Equis.
  • "Democrats should be giving Trump credit for the control and the maintenance of the U.S.-Mexico border," she said. But "at the end of the day, people are asking for a lot more than that."