President Trump walks from the Oval Office to the Marine One helicopter as he departs from the South Lawn at the White House on July 31. (Jabin Botsford/The Washington Post)

President Trump’s conservative supporters are spoiling for a fight over funding for his border wall this fall even if it risks a government shutdown, setting up a stark choice for the White House: Juice the president’s base or placate Republicans who fear the electoral fallout.

Trump is under mounting pressure from the far right to act aggressively on his top campaign promise — which remains unfulfilled 18 months into his presidency — despite warnings from GOP leaders in Congress that a budget standoff would sow chaos just weeks before an already tenuous midterm election.

But Trump’s hardcore backers contend that the president has more to gain by satisfying them than worrying about alienating more-moderate voters.

“Do you think there’s a single person who voted for Trump on his signature issue who says it’s not worth shutting the government down, who says I’m not voting for Republicans because of it?” said Lars Larson, a conservative talk show host based in Portland, Ore. “I think Trump should take a hard line on this. Let’s get that wall.”

The decision highlights the fraught role of immigration in the nation’s political debate and the tricky calculus for the Trump White House as Democrats already hold a polling edge on November’s generic House ballot.

In some ways, Trump’s predicament offers an echo of 2014, when President Barack Obama deferred to the wishes of wary Senate Democrats and delayed a major expansion of a program to protect undocumented immigrants until after the midterms. Perhaps instructive for Trump, Democrats lost nine seats and control of the Senate that year, and Obama was criticized by some in his base for the strategy.

Trump on Wednesday alluded to the competing pressures during an interview with conservative host Rush Limbaugh, acknowledging that a pre-election shutdown is “riskier” but suggesting he would give up leverage by waiting. A House spending bill has dedicated $5 billion for the wall along the U.S.-Mexico border, but a Senate plan offers only $1.6 billion.

“I’m just not sure you’d make as good a deal,” Trump said of waiting until after the election. “I actually think it’s a great campaign issue. I think it would be great before [the election]. But I don’t want to disappoint a lot of very good people that are working with me” among the GOP leadership.

Republican leaders in recent days have expressed confidence that Trump understands the electoral stakes and will hold off until after the midterm for the showdown on the border wall to ease pressure on GOP candidates in tight races in suburban districts. Polls show a majority of the country opposes the wall.

But a growing chorus of conservative commentators, including Ann Coulter and Ben Shapiro, have suggested that fears of a shutdown rebounding negatively on Republicans and accelerating a blue wave in November are overblown.

Rep. Jim Jordan (R-Ohio), a member of the Freedom Caucus who has announced a bid for House speaker if Republicans maintain control, told the Conservative Review that holding off on the wall funding fight is “just the opposite of what we should be doing.”

“Do we want to nationalize these elections and fire up Republican and Trump voters to come out and vote for us?” Jordan said. “Then we better just not kick the can past the elections. We better actually do the things we said we were going to do.”

That was the same argument some liberals made in summer 2014 after the collapse of a comprehensive immigration bill in the GOP-controlled House. Obama vowed to act unilaterally by summer’s end to expand deportation protections for millions of undocumented immigrants.

But in September, Obama announced he would hold off until after the midterm elections, amid fears from Senate Democrats that a polarizing immigration announcement would harm incumbents on the ballot in North Carolina, Louisiana, Colorado and Arkansas.

Obama and his aides argued that the new policy would be more “sustainable” if it were separated from the heat of the political season. They accused Republicans of politicizing a crisis that summer in which tens of thousands of unaccompanied minors from Central America had crossed the U.S.-Mexico border without authorization.

In the wake of a decisive GOP victory that fall, however, some Democrats suggested that Obama’s immigration action — which he ultimately announced two weeks after the election — would have rallied Latinos and other liberal groups at the polls.

A former campaign aide to Democrat Mark Udall, who lost his Senate reelection bid in Colorado, recalled in an interview Wednesday that Democrats were facing head winds because of perceived national security threats, including the rise of the Islamic State and an Ebola scare in Africa, as well as the border crisis.

“Anything that would have encouraged likely Democratic voters to be excited about anything would have been good,” said the former aide, who spoke on the condition of anonymity because he was not authorized by his employer to comment on the record. “If it was a political calculation that delayed that announcement, it was misguided, or at least obviously it didn’t work.”

Senate Majority Leader Mitch McConnell (R-Ky.) and House Speaker Paul D. Ryan (R-Wis.) have presented Trump with a plan to move several piecemeal spending bills ahead of the Sept. 30 deadline to fund most governmental functions, but to postpone a vote on the Department of Homeland Security’s budget, and a potential fight over the wall, until after the midterms.

Dale Jackson, a conservative talk radio host in Huntsville, Ala., said the GOP leadership is right to worry about the political harm of a shutdown, and he blamed Trump for not acting sooner to secure wall funding. But Jackson acknowledged that Trump’s supporters “would love to see progress toward a wall — they don’t care too much about a government shutdown.”

Others suggested that the benefits for Trump in confronting Democrats would extend even beyond his base.

“If the president is seen as weak on a signature issue, that doesn’t just hurt with the base but with lots of people,” said Mark Krikorian, executive director of the Center for Immigration Studies, which advocates reducing immigration levels. “Even people who aren’t fired up about it are at least going to be able to say they know what he thinks about immigration. If he starts wimping out, he loses credibility even among people who aren’t in his base.”

Correction: An earlier version of this story inaccurately stated that the Senate’s spending bill did not permit new border barriers. It has been corrected.