New York (CNN Business) Democratic presidential candidate Elizabeth Warren wants the SEC to crack down on credit rating agencies Moody's and Standard & Poor's for what she calls "troubling reports of inflated bond ratings and the perverse incentives within the bond rating industry."

Warren, the Massachusetts senator and a frontrunner for the 2020 Democratic nomination, has been a frequent critic of corporate malfeasance.

In a letter sent to Securities and Exchange Commission chair Jay Clayton on Thursday, Warren said that even though reforms were put into place after the 2008 financial crisis to increase oversight of the ratings agencies, "the SEC and other federal agencies have neglected or delayed implementation of many of the changes."

She said a big problem is that companies pay Moody's and S&P to rate their bonds.

"Under the issuer-pays model, bond issuers - like large financial institutions - pay the agencies for their assessments of the products they hope to sell, ultimately giving the rating firms an incentive to give better ratings, regardless of the risk, since bond issuers might otherwise go to their competitors," Warren wrote.

Read More