Liberal activist and filmmaker Michael Moore likened the GOP tax law to an "act of terror," saying it will make life harder for many Americans.

"This hurts this country," Moore said Thursday on MSNBC's "The Last Word." "This tax cut [was] in a sense an act of terror, because it’s going to make the people who are already struggling to get by that much harder to get by.

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“And to alleviate their pain, how many more of those opioid drugs are they going to need?" he continued. "What will happen to their children in their inadequate schools, the air they’re breathing, the climate change they don’t believe in, all of that.”

Moore made the remarks after watching footage of Republican lawmakers celebrating as President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE signed the tax-reform package into law in December 2017.

“All the Republicans in that photo you showed standing there on the steps with him a year ago today, all looking so slap-happy," Moore told host Ali Velshi, who was filling in for Lawrence O'Donnell. "Remember, it’s not just Trump, but all of the Republicans, all of them. Nobody has had the courage to stand up and say this is wrong, this is un-American."

Moore also addressed the resignation of Defense Secretary James Mattis James Norman MattisBiden courts veterans amid fallout from Trump military controversies Trump says he wanted to take out Syria's Assad but Mattis opposed it Gary Cohn: 'I haven't made up my mind' on vote for president in November MORE, who was seen as a counterweight to Trump's unorthodox foreign policy.

“Trump is going to put in the person that agrees with him and is gonna be his dog,” Moore said. “We have an autocrat in the White House.”

Mattis announced his resignation on Thursday, citing differences with the president on foreign policy.