U.S. President Donald Trump on Monday took aim at the nation's health insurers in an escalating threat to cut the health care subsidy payments that make Obamacare plans affordable, one day after urging Republican senators to continue working to undo his Democratic predecessor's health care law.

"If ObamaCare is hurting people, & it is, why shouldn't it hurt the insurance companies & why should Congress not be paying what public pays?" Trump, a Republican, wrote on Twitter.

Trump, frustrated that he and Republicans have not been able to follow through on their campaign promises to repeal and replace Obamacare, has repeatedly threatened to let it implode. So far, the administration has continued to make the monthly subsidy payments, and withholding them is just one way they could make good on Trump's threat that could weaken the law.