A group of high-profile Democratic Senators, led by Sherrod Brown of Ohio, wrote letters to the nation’s top retail CEOs Monday, demanding they crack down on trucking companies that turned their workers into modern-day indentured servants.The call to action comes in response to a yearlong USA TODAY Network investigation that found port trucking companies in California forced their drivers into debt, pressured them to work up to 20 hours a day and paid them pennies per hour.”As a major U.S. corporation,” the senators wrote, “you also have a role to play in ensuring that you are not complicit in the mistreatment of port truck drivers and that American consumers, your customers, are not unwittingly supporting labor abuses in the United States.”More from USA Today: How trucking companies forced drivers into debt, worked them past exhaustion and left them destituteNuclear labs endanger public with radioactive mailVenezuelan opposition leaders taken from their homes by state security agentsSen. Elizabeth Warren, D-Mass., and Sens Dianne Feinstein and Kamala Harris of California joined Brown in sending the letter to 16 retail brands, including Target, Wal-Mart and Costco. They asked each CEO to answer a series of questions about their knowledge of labor violations in the port trucking industry and about their plans to cut ties with those companies.Together, these four senators represent powerful voices around the issues of labor, immigration and corporate malfeasance.Warren, of Massachusetts, famously took on Wall Street after the real estate collapse in 2008 and transformed her popularity as a consumer crusader into a Senate bid. California Sen. Feinstein is the ranking Democrat on the powerful judiciary committee.”Port trucking companies’ brazen disregard for federal transportation safety standards and workers’ safety and rights is shameful,” they wrote in Monday’s letter.They pledged to “pursue aggressively all federal avenues to put an end to this rampant mistreatment of port truck drivers.””We cannot allow corporate profits to come at the expense of American workers, and that means putting an end to abusive practices we’ve seen in the port trucking industry,” Brown said in a statement.Since USA TODAY Network published its investigation in June, elected officials across the country have been pressing for action.Dozens of lawmakers, from the Los Angeles City Council to Capitol Hill, have denounced the business practices, many calling on retail corporations to better police their trucking contractors.Los Angeles City Councilman Joe Buscaino wrote a letter to the CEO of Target, suggesting the retail giant adopt a code of conduct specific to its port trucking contractors.In a follow-up statement to reporters, Buscaino said he wants a “commitment from the big box retailers to stop doing business with trucking companies that treat their drivers as indentured servants.”Target, which previously called the mistreatment of workers in its supply chain “unacceptable,” declined to comment about the letters. Others, contacted Monday afternoon by USA TODAY, did not have an immediate response.As they investigated the port trucking industry, reporters contacted two dozen retail companies, including Target, Costco and LG, that have used port trucking companies with labor violations. The few…