Former Democratic National Committee Chairman Howard Dean on Sunday called President Trump Donald John TrumpSteele Dossier sub-source was subject of FBI counterintelligence probe Pelosi slams Trump executive order on pre-existing conditions: It 'isn't worth the paper it's signed on' Trump 'no longer angry' at Romney because of Supreme Court stance MORE "mentally incapacitated," and said Democrats can't negotiate with someone "irrational" to end the partial government shutdown.

"I actually think the president is mentally incapacitated and I have thought so for quite some time," Dean, a frequent critic of Trump's known for his polarizing comments, said on MSNBC.

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"He says things that he knows aren’t true I suspect and that somehow he thinks that people are going to believe. And they just don’t."

"You can’t really make a bargain with somebody who is irrational, because they won’t keep it," Dean went on to say. "The Republicans and the Democrats in Congress have got to do the best they can to run this country in the absence of the chief executive. Until the Republicans realize they have an obligation to the country, not just their party, that’s not going to happen."

The partial government shutdown began on Dec. 22 and shows no signs of ending.

Trump wants a funding bill to include $5 billion for a wall on the Mexican border, which Democrats oppose.

Both sides continue to dig in over the issue days before a new Congress — and a Democratic majority in the House — is set to come to Washington.

"I campaigned on Border Security, which you cannot have without a strong and powerful Wall," Trump tweeted early Monday morning. "Our Southern Border has long been an 'Open Wound,' where drugs, criminals (including human traffickers) and illegals would pour into our Country. Dems should get back here [and] fix now!"