President Trump on Monday escalated a feud with a veteran Democratic lawmaker who called for aggressive protests of administration officials, warning Rep. Maxine Waters of California to “Be careful what you wish for Max!” even as leaders in both parties cautioned against increasingly caustic political rhetoric.

Trump’s message, conveyed on Twitter, stood in sharp contrast to a call for civility by White House press secretary Sarah Huckabee Sanders, whose ouster from a Virginia restaurant sparked a furious debate over whether opponents should take out their political frustrations against administration officials in their private lives.

The tensions have flared as part of an acrimonious standoff over immigration and the separation of migrant families at the U.S.-Mexico border, which has prompted some protesters to hound Republican officials outside their homes, in restaurants and at movie theaters. Sanders was asked to leave the Red Hen restaurant in Lexington, Va., on Friday night because the owner said she believes Sanders works for an “inhumane and unethical” administration.

Stephanie Wilkinson, owner of the Red Hen restaurant in Lexington, Va., asked White House press secretary Sarah Huckabee Sanders to leave on June 22, 2018. (Allie Caren, Dalton Bennett/The Washington Post)

Democratic fears of a backlash have spilled into public view since Waters, a vocal Trump critic, told supporters at a Los Angeles rally Saturday that “if you see anybody from that Cabinet in a restaurant, in a department store, at a gasoline station, you get out and you create a crowd and you push back on them!” She repeated that call in an MSNBC interview later the same day.

In a rare rebuke of a fellow Democrat, House Minority Leader Nancy Pelosi (D-Calif.) criticized Waters’s comments Monday.

“In the crucial months ahead, we must strive to make America beautiful again,” Pelosi wrote in a tweet. “Trump’s daily lack of civility has provoked responses that are predictable but unacceptable. As we go forward, we must conduct elections in a way that achieves unity from sea to shining sea.”

Some Democrats worry that the remarks by Waters, coupled with the ejection of Sanders, are an unneeded distraction at a time of swelling public anger over reports of desperate children being separated from their parents because of the Trump administration’s “zero-tolerance” immigration policy.

Rep. Maxine Waters (D-Calif.) encouraged supporters at a rally in Los Angeles June 23 to stand up to members of President Trump’s administration. (Allie Caren, Monica Akhtar/The Washington Post)

On Monday afternoon, Trump responded to Waters with insults. In a tweet, Trump called Waters “an extraordinarily low IQ person” who, together with Pelosi, had become “the Face of the Democrat Party.” He also appeared to issue a veiled threat to Waters herself.

“She has just called for harm to supporters, of which there are many, of the Make America Great Again movement,” Trump wrote. “Be careful what you wish for Max!”

Waters disputed Trump’s characterization of her remarks.

“I believe in peaceful, very peaceful protest,” she told reporters on Capitol Hill, according to video footage broadcast by CNN. “I have not called for the harm of anybody. This president has lied again when he’s saying that I called for harm to anyone.”

Later, at Monday’s White House press briefing, Sanders referred to the Red Hen incident and said that all Americans should be allowed to disagree “freely and without fear of harm.”

“Healthy debate on ideas and political philosophy is important, but the calls for harassment and push for any Trump supporter to avoid the public is unacceptable,” Sanders said. “America is a great country, and our ability to find solutions despite disagreements is what makes us unique.”

Both parties have grappled with threats in recent years amid political firestorms. In the wake of the passage of President Barack Obama’s health-care law, Democratic lawmakers received death threats. Wisconsin Gov. Scott Walker (R) said his family was threatened when he clashed with supporters of public labor unions in 2011.

Trump has also been accused of stoking violence through his language at rallies. In 2016 in Iowa, for example, he said that if someone decided to “knock the crap out of” a protester, he would “pay for the legal fees, I promise.”

Adding to the mixed messages, the National Republican Senatorial Committee sent out an email on Monday with the subject line, “Wacky Jacky’s delusional world view,” using Trump’s mocking nickname for Rep. Jacky Rosen, the Democratic Senate nominee in Nevada.

Waters has long been a hero to some Democrats and a villain to some Republicans — and her clashes with Trump have elevated her profile further. “I don’t honor him. I don’t respect him, and I don’t want to be involved with him,” Waters said in January 2017.

Former House speaker Newt Gingrich, a Trump ally, described the Waters-Trump standoff as another instance of elements of the Democratic Party being outraged by Trump’s election in 2016, which could play to Trump’s advantage.

“If the country looks up at the television and sees Maxine Waters saying what she said, that Democrats should make life miserable for the Trump administration, most voters will say that’s not right,” he said.

But others said the Trump wing of the GOP should not be too encouraged.

“It doesn’t mean anything. It’s the tribal outrage of the week,” said veteran Republican strategist Mike Murphy, a Trump critic. “Each tribe feeds itself, and the circus eventually moves on to something new. These are cheap calories and won’t move the needle very much.”

While Democrats feel increasingly confident about their chances in this fall’s midterm elections, they are also being pulled into near-daily political riptides by a president who has thrived on controversy for decades.

One Democratic leadership aide described the Waters comments as “a gift” to Republicans.

“She went too far. Her comments, to an impartial observer, came off as a call for violence, and that is too far,” said the aide, who was not authorized to speak publicly about the issue. “It became a distraction and, obviously, it needed to be addressed.”

Democratic strategist Paul Begala warned that overly aggressive tactics could backfire by alienating the voters that his party needs most.

“My aversion to answering right-wing hate with left-wing hate is strategic as well as moral,” Begala wrote in a Monday afternoon tweet. “The R’s are trying to keep older, high school educated white men angry. I want to show college-educated GOP women there’s no home for them in vulgar, vicious Trumpland.”

Several congressional Democrats issued clear rebukes of Waters’s remarks, although they did not mention her by name.

“No one should call for the harassment of political opponents. That’s not right. That’s not American,” Senate Minority Leader Charles E. Schumer (D-N.Y.) said in a speech on the Senate floor. He added: “The president’s tactics and behavior should never be emulated. It should be repudiated.”

Sen. Cory Booker (D-N.J.), appearing on MSNBC, urged protesters to follow the example of the Rev. Martin Luther King Jr. and other civil rights leaders “who always did it by recognizing the dignity of even those who you oppose, even those who are trying to destroy you, even those that hate you.”

More broadly, there is concern among some veteran Democrats that Republican turnout in November’s midterms could be stoked by the issues being targeted by various Democratic activists, such as the ongoing Russia probe by special counsel Robert S. Mueller III and calls for Trump’s impeachment.

“The only way we lose the House is if we do something to rile up the Trump base,” Ed Rendell, a former Democratic National Committee chairman, said in an interview earlier this month. “Right now they’re a little riled, and the worst thing we could do is talk about impeachment. I’d like to kidnap Tom Steyer and hold him in Bolivia until after the election.”

Steyer, a California billionaire, is funding a national television advertising campaign that calls for Democrats to rally behind impeaching Trump.

Former Vermont governor Howard Dean (D), who ran for president in 2004 amid antiwar fervor in the party and later served as national party chair, dismissed the latest controversy as a “Beltway game” — and predicted that Democrats would do well this year because of bigger questions facing voters.

“It’s not about Maxine Waters, it’s now about the survival of the country,” Dean said in an interview. “Trump’s provocations are irrelevant, and more people are just not interested in them, especially people under 40. They’re focused on building a different kind of America that embraces people and doesn’t appeal to hate.”

Sean Sullivan and Philip Rucker contributed to this report.