President Trump's claim Tuesday that the federal response to Puerto Rico was an "unsung success," after the governor of Puerto Rico increased the official Hurricane Maria death toll from 64 to 2,975, concerned some in Congress. But now the president's denial of the death count altogether, claiming the other party inflated the figure to smear him, has elicited outrage from Democrats.

The estimate of 2,975 excess deaths from September 2017 to February 2018 was reached by George Washington University researchers in an independent study, and its findings were adopted by Puerto Rico's governor. The president's denials came as residents of North and South Carolina and neighboring states were bracing for the arrival of Hurricane Florence.

"3000 people did not die in the two hurricanes that hit Puerto Rico. When I left the Island, AFTER the storm had hit, they had anywhere from 6 to 18 deaths. As time went by it did not go up by much. Then, a long time later, they started to report really large numbers, like 3000," the president tweeted Thursday. "...This was done by the Democrats in order to make me look as bad as possible when I was successfully raising Billions of Dollars to help rebuild Puerto Rico. If a person died for any reason, like old age, just add them onto the list. Bad politics. I love Puerto Rico!"

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Democrats slammed the president's remark and also lashed out at Republicans, who currently run all oversight committees in both the House and Senate.

House Minority Leader Nancy Pelosi said the president is using "alternative facts," and claimed Republicans are "determined to shield" his administration's failures.

"@realDonaldTrump prefers his 'alternative facts' to the tragedy faced by families of the lost. Worse still, the GOP is determined to shield his insulting behavior from accountability," she tweeted Thursday morning. "It's time for Republicans in Congress to get back to performing our crucial oversight function."

Sen. Bob Menendez, D-New Jersey, tweeted, "You're right, Mr. President. The Hurricane didn't kill 3,000 people. Your botched response did."

You’re right, Mr. President. The Hurricane didn’t kill 3,000 people. Your botched response did. https://t.co/PifAMD9aaE — Senator Bob Menendez (@SenatorMenendez) September 13, 2018

Rep. Bennie G. Thompson, D-Mississippi, the ranking member of the House Homeland Security Committee, said the president's failure to accept responsibility "shows us, once again, that he is not fit to serve as our president," and called on Mr. Trump to resign. He also called out his GOP colleagues.

"He should resign at once — and this Republican-led Congress must stop being complacent and finally conduct a comprehensive investigation — as we did after Katrina — into how 2,975 were allowed to die," Thompson said in a statement.

The House Democratic Caucus tweeted, "President Trump won't acknowledge the thousands of Americans who died on his watch. And even worse, Republicans have no interest in holding this administration accountable and ensuring that Congress is prepared to respond to these disasters."

Sen. Claire McCaskill, D-Missouri, dismissed the president's claim that Democrats inflated the death toll.

"The study that was done on the death toll was done by a respected American university not the Democratic Party," she said.

Sen. Doug Jones, D-Alabama, called the president "dead wrong."

"It's just frustrating when he denies reality sometimes to protect himself," Jones said.

Republicans were slower to respond to the president's death toll claim, although Republican Speaker of the House Paul Ryan said he has no reason to dispute the higher death count numbers.

"Casualties do not make a person look back," Ryan said. "That's not — so I have no reason to dispute these numbers. I was in Puerto Rico after the hurricane and it was devastated."

When Sen. Orrin Hatch, R-Utah, was asked for a response on Mr. Trump's tweets, he said he knew nothing about them.

"Wait say that again? I don't know anything about it," he told reporters on Capitol Hill.

CBS News' Alan He contributed to this report.