About 6 in 10 undocumented immigrants in the United States live in one of 20 metro areas, with three regions in Southern California combining to make up the largest cluster of people living in the country illegally.

The high concentration of undocumented immigrants in these metro areas suggests that the impact of President Donald Trump’s immigration policies–including his plans to ramp up deportations–will be most widely felt in the nation’s largest cities.

The study also shows that a big swath of the nation’s undocumented population lives in communities that voted against Trump in the last election, which could minimize the political fallout for the White House as it moves to implement some of the president’s hardline immigration agenda.

Pew, which has tracked the undocumented population for several years, pegs the total number of undocumented immigrants at about 11.1 million nationwide.

In Southern California, about 1.4 million undocumented immigrants live between the southern Ventura County border and the U.S.-Mexico border — the biggest cluster of undocumented people in the country.

Overall, undocumented immigrants make up about 3.5% of the nation’s population. Pew noted that some cities with the highest concentration of undocumented residents include Houston (8.7%) and Las Vegas (8%).