Donald Trump supporters across the US have lashed out against his proposal to create a path to citizenship for nearly two million "Dreamer" immigrants.
Fearing betrayal on a signature campaign issue, Trump-aligned candidates from Nevada and Virginia rejected the notion outright.
The president’s most loyal media ally, Breitbart News, attacked him as "Amnesty Don", and outside groups who cheered the hardline rhetoric that dominated Mr Trump’s election campaign warned of a fierce backlash against the president’s party in November’s mid-term elections.
"There’s a real potential for disaster," said Mark Krikorian, executive director of the far-right Centre for Immigration Studies.
"The president hasn’t sold out his voters yet. But I think it’s important that his supporters are making clear to him that they’re keeping an eye on him."
The public scolding was aimed at a president who has changed course under pressure before, but Mr Trump has faced no greater test on a more significant issue than this one, which dominated his outsider candidacy and inspired a coalition of working-class voters that fuelled his unlikely rise.
Now, barely a year into his presidency, he can bend either to the will of his base or the pressure to govern and compromise.
His leadership may determine the fate of hundreds of thousands of young immigrants and whether his party can improve its standing among a surging group of Hispanic voters. It may also alienate those who love him most.
"There’s a Trump movement. And it’s not necessarily about Donald Trump," said Corey Stewart, a Republican Senate candidate in Virginia and a vocal Trump ally.
"It’s about the things that Donald Trump campaigned and stood for during his campaign. Ultimately, every elected leader needs to stay true to the message that they ran on, otherwise people will leave them."
The response underscores the Republican Party’s immigration dilemma in the age of Trump.
Much of the country, including independents and moderate Republicans, favour protections for thousands of young people brought to the country as children illegally and raised here through no fault of their own.
But a vocal conservative faction emboldened by Mr Trump’s anti-immigrant rhetoric will never accept anything viewed as "amnesty", and many view legal protection for these young immigrants as just that.
Mr Trump’s proposal includes billions for border security and significant changes to legal immigration long sought by hardliners.
Several Democrats and immigration activists rejected it outright, but his supporters’ focus on "amnesty" for Dreamers highlights how dug in the base is and how little room he has to manoeuvre.
The president told reporters this week that he favoured a pathway to citizenship for those immigrants, embracing a notion he once specifically rejected.
Legal protection for roughly 700,000 immigrants enrolled in the Deferred Action for Childhood Arrivals programme, or DACA, has emerged as the driving priority for Democrats, who forced a government shutdown over this issue last week.