Sen. Bernie Sanders (I-Vt.) warned President-elect Donald Trump that “we will be his worst nightmare” if Trump directs the anger of his supporters toward Muslims, Hispanics, black people and women.

In a statement Wednesday, Sanders acknowledged that Trump “tapped into the anger of a declining middle class.” But on Thursday, Sanders issued a warning:

If Donald Trump takes people's anger and turns it against Muslims, Hispanics, African Americans and women, we will be his worst nightmare. — Bernie Sanders (@SenSanders) November 10, 2016

Sanders said he and other progressives are prepared to work with the president-elect on “pursuing policies that improve the lives of working families in this country.”

The senator, who ran in the Democratic presidential primary before endorsing former Secretary of State Hillary Clinton, has long been a critic of Trump.

Trump visited Capitol Hill on Thursday for the first time as president-elect to meet with House Speaker Paul Ryan (R-Wis.) and Senate Majority Leader Mitch McConnell (R-Ky.).