A bipartisan group of 16 mayors from across the country took a bus to the Mexican border last week to bring awareness to family separations taking place under the Trump Administration’s zero-tolerance policy.

It was an unusual undertaking for a group of mayors, who are typically more concerned with potholes than national policy.

But experts say that Donald Trump’s presidency is accelerating a trend that had begun even before he took office of local officials getting more involved in national issues. All politics is no longer just local, it seems.

“I think what you’re seeing is the power of mayors coming together,” said Miami Mayor Francis Suarez, a Republican who recently criticized the family separation policy in a letter to the editor in the Miami Herald. “We as cities contain about 80 to 85% of the entire population of the U.S. in our cities. … We can start having a bigger voice as a national group.”

It’s not just the family separation policy. Local elected officials have also pushed back against Trump by declaring their areas “sanctuary cities,” where local police will not report potential immigration violations to the federal government; passing resolutions condemning Trump policies; and declining a traditional visit to the White House.

One reason for the trend is the increasing power that Democrats wield in bigger cities. Pushing back against an unpopular Republican president can win local support, which is especially crucial in areas where the races are essentially decided in party primaries.

“Many of the big cities are so Democratic that the real contest is among Democratic candidates,” notes John Kincaid, a professor of at Lafayette and the director of the Meyner Center for the Study of State and Local Government.

For mayors who are looking to run for higher office, picking a fight with the president can also be a way to move onto bigger issues. In the current Congress, there are 43 former mayors — 35 in the House and 8 in the Senate — a total of about 8%.

Steve Benjamin, the mayor of Columbia, South Carolina, and an organizer of the border bus tour, said he’s trying to reflect the thoughts of his constituents.

“My city is 43% African American and I’m a black mayor, but we have citizens from 200 different countries that speak 90 different languages,” he said. “We’re a microcosm of a global society.”

Other mayors said they were moved to act by the seriousness of the family separation issue.

Mayor Keisha Bottoms of Atlanta, who signed an executive order that would bar Atlanta’s city jail from accepting Immigration and Customs Enforcement detainees, noted that her seven-year-old daughter was reading a news article about family separations over her shoulder recently.

“She asked me ‘What does sobbing mean?’ because there was a story about children sobbing after being separated from their parents,” she said.

Bottoms said that spurred her to do more.

“Watching the news over the past few weeks and watching these families separated in this very inhumane way really made me think about what it would be like when I look into the faces of my children and say what we did as a city,” she said. “I wanted to make sure that we weren’t inadvertently complicit in this policy.”

Kincaid said this kind of mayoral activism has deep historical roots, noting that some local officials in the antebellum era avoided efforts to help authorities catch runaway slaves, while more recently city councils declared themselves nuclear-free zones or divested from South Africa during Apartheid.

Austin, Texas Mayor Steve Adler, who endorsed and helped to pass a resolution that requires police to inform individuals of their right to withhold their immigration status from officers, said it’s just the nature of the job to try to do something.

“People expect their city government to actually solve problems, not to get lost in debates. They expect mayors to perform,” he said. “At the local level, when I go to the grocery store, I’m surrounded by my constituents telling me what they want to happen.”

Benjamin agreed. “People in Washington like to talk a lot,” he said. “Mayors act.”

Correction: June 29

The original version of this story misspelled Miami Mayor Francis Suarez’s first name. It is Francis, not Frances.

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