Rep. Henry Cuellar (D-Tex.) sent a letter to Vice President Harris on Tuesday urging her to visit the southern border with him and telling her that an increase in migrants has caused “severe burdens” — a move that puts fresh pressure on Harris from within her own party to deliver on her recent promise to travel to the region.

“I write to invite you to the U.S. Southern Border to observe the ongoing humanitarian crisis and share the perspective of Americans who live there. I encourage you to join me and other Members of Congress, while we visit with the people on the ground who deal with these issues every day,” writes Cuellar in the letter, which he shared with The Washington Post.

Cuellar also writes that the “rapid rise in the number of migrants arriving at our Southern Border has placed severe burdens on our communities.” He mentions the challenges posed by the pandemic, economic struggles and travel restrictions, before concluding: “The situation on the border will only worsen, as more migrants are expected to arrive. The Administration needs to take a proactive approach to create a sustainable system of humanitarian aid and relieve everyday Americans of that responsibility.”

The letter, which echoes Cuellar’s recent public comments, comes a week after Harris concluded a trip to Mexico and Guatemala aimed at addressing the underlying reasons many people are fleeing to the United States, often under dangerous circumstances. While abroad, Harris said she would travel to the U.S.-Mexico border at some point. But she didn’t specify when that would happen or under what circumstances.

Cuellar’s letter underlines the concerns President Biden and Harris are confronting from some Democrats about addressing the surge of migrants at the border. These worried Democrats are urging the administration to demonstrate their commitment to the issue in a visible way. For centrist Democrats like Cuellar, whose district includes hundreds of miles along the border with Mexico, the migrant surge has become a political and policy liability that Republicans have pounced on in recent months.

The congressman from Texas has been one of his party’s most outspoken critics of the Biden administration’s border and immigration policies, at times publicly challenging it to step up enforcement and intensify efforts to deter people from coming. He has previously been vocal about challenging Biden and Harris to come to the border and has suggested the administration dissuade migrants from coming by widely promoting images of people being turned away.

Former president Donald Trump announced Tuesday that he planned to travel to the border with Texas Gov. Greg Abbott, a fellow Republican, on June 30. “What Biden and Harris have done, and are continuing to do on our border, is a grave and willful dereliction of duty,” Trump said in a statement.

U.S. authorities intercepted more than 180,000 migrants along the border in May, according to U.S. Customs and Border Protection data published last week that showed illegal crossing had reached a new 20-year high.

A Harris spokeswoman did not immediately respond to a request for comment on Cuellar’s letter.

A Harris spokeswoman confirmed receipt of the letter but declined to comment on it otherwise.

In late March, Biden tapped Harris as his point person for overseeing efforts to slow the increase of migrants. She initially struggled to define her role, emphasizing that she was focused on the circumstances in Latin America that are driving people to come, rather than the day-to-day management of the border — all as Republicans disparagingly sought to brand her Biden’s “border czar.”

During Harris’s trip to Latin America last week — her first foreign travel as vice president — she faced questions from reporters about why she had not gone to the U.S.-Mexico border. Her handling of the questions prompted Republicans to escalate their criticism and point to an exchange in an interview with NBC News that received widespread attention.

“At some point, you know, we are going to the border,” Harris said in that interview. “We’ve been to the border. So this whole thing about the border — we’ve been to the border.”

NBC’s Lester Holt noted that she hadn’t been to the border since becoming vice president.

“And I haven’t been to Europe,” Harris replied. “And I mean, I don’t understand the point that you’re making. I’m not discounting the importance of the border.”

Later, Harris reiterated to reporters that she would head to the border. “Yes, I will and I have before,” she said. Harris did not specify a date.

Even as Harris has faced concerns from some moderate Democrats that she has not addressed the migration surge aggressively enough, liberals have at times been critical of what they see as too harsh a tone. Some were unhappy when Harris told would-be migrants not to make the trek to the United States. “Do not come,” she said during her trip, echoing a refrain Biden and other top administration officials adopted earlier this year.

“This is disappointing to see,” Rep. Alexandria Ocasio-Cortez (D-N.Y.) tweeted in response to Harris’s comment.