Back Wednesday from her debut international mission as US vice president, Kamala Harris caught flak from Republican critics accusing her of not taking the migration crisis seriously enough, but also from vocal progressives frustrated about border policy.

The two-sided pile-on reflects the protracted and difficult nature of America’s immigration problem, which has vexed US administrations for decades.

But Harris’ trip to Guatemala and Mexico, during which she appeared to flippantly respond to questions about why she has yet to visit the southern US border for an on-the-ground assessment, marks her biggest stumble yet since she became President Joe Biden’s deputy.

Senator John Cornyn, a Republican from Texas, described her trip as a “missed opportunity.”

“She is avoiding the point of failure in our immigration system, which is at the border,” Cornyn told AFP Wednesday in the US Capitol.

Less than a decade ago then-president Barack Obama faced a humanitarian crisis at the border, and struggled to apply the most effective response to thousands of people — including unaccompanied minors — fleeing violence and poverty in their home countries.

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His successor Donald Trump famously vowed to build a border wall, and earned opprobrium from critics for controversial detentions.

Now Biden has faltered on handling illegal immigration, and critics smell blood in the water — particularly after a series of Harris responses to questions about why she has yet to visit the US-Mexico border since the president tasked her with the immigration portfolio.

In Guatemala she said she would remain focused on addressing the root causes of illegal migration — poverty and crime — rather than the “grand gestures” of a hypothetical border visit.

In an interview with NBC things grew tense. When Harris said “we’ve been to the border,” the reporter noted that she had yet to visit in person as vice president.

“And I haven’t been to Europe,” she retorted, before breaking into a nervous laugh. The exchange brought fierce criticism from Republicans.

Detentions of undocumented travellers along the US-Mexico border, including unaccompanied minors, hit a 15-year high in April.

Nearly 180,000 people were intercepted — more than 80% of them coming from Mexico or the so-called Northern Triangle of Guatemala, Honduras and El Salvador.

Harris arrived Sunday in Guatemala, where she levelled a firm warning to would-be migrants: “Do not come” to the United States.

But she also brought the Biden administration’s pledge of a more humane immigration policy — in stark contrast with Trump’s “zero tolerance” approach which is favored by Republicans.

“Everything Joe and Kamala have done has sent exactly the opposite message” from Harris’ “do not come” remark, Republican Senator Ted Cruz told Fox News on Tuesday.

“We’ve seen in the five months they’ve been in office, an absolute disaster unfolding at the border” with no willingness to fix the problem, the senator from Texas said.

He noted the surge in undocumented arrivals, Biden’s ending of his predecessor’s “Remain in Mexico” program, and the refusal to stop the practice of catch and release for low-risk migrants allowed to stay in the United States pending immigration hearings.

Harris “literally flew twice over our southern border,” another Republican, Senator James Lankford, lamented to AFP, saying the vice president should have stopped “to get a read on the ground of what’s actually happening.”

While Harris urged migrants against making the perilous journey northward, Biden’s “policies have spoken the opposite,” Lankford noted.

But those same “do not come” remarks by Harris antagonized those on her party’s left flank.

“Disappointing” is how liberal House Democrat Alexandria Ocasio-Cortez described Harris’ warning.

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“This whole ‘stay there and die’ approach is not how our country will promote a more humane and just immigration system,” congresswoman Rashida Tlaib tweeted.

The White House meanwhile offered Harris its languid support, saying her trip to the Northern Triangle to address root causes of illegal migration was “exactly what the president asked her to do.”

Democratic Senator Tim Kaine, who chairs the Western Hemisphere Subcommittee on Foreign Relations, said it was “really important” that Harris visited Guatemala and Mexico as a way to show the administration’s commitment to tackling illegal immigration.

Kaine downplayed the criticism of Harris, who once served as California’s attorney general, as unfair opportunistic denunciations.

“I imagine she knows a lot more about the border than some of the folks that are criticizing her,” Kaine said.