A former labor secretary is calling on the 420,000 federal employees who are working without pay due to the shutdown to walk off the job and picket outside the workplace.

Robert Reich, a University of California, Berkeley professor who served in President Bill Clinton’s administration, slammed President Donald Trump for his “callous disregard” of the federal workforce:

As a former secretary of labor, I urge federal workers who continue to toil without pay to stop working and those who are furloughed to picket their workplaces. Trump's callous disregard for all of you and for the public welfare must end. He must reopen the U.S. government. — Robert Reich (@RBReich) January 22, 2019

The government shutdown reached the one-month mark this week, with many workers set to miss their second paycheck on Friday.

In addition to the 420,000 who are working without pay, an additional 380,000 have been furloughed and are also missing paychecks.

Congress passed a bill granting them backpay, but it will not be paid out until the shutdown ends.