Retired Gen John Kelly, previously the Homeland Security secretary, takes over today from the ousted Reince Priebus.
Trump hopes Kelly can bring some military order to an administration weighed down by a stalled legislative agenda, a cabal of infighting West Wing aides and a stack of investigations.
Still, Kelly's success in a chaotic White House will depend on how much authority he is granted and whether Trump's dueling aides will put aside their rivalries to work together. Also unclear is whether a new chief of staff will have any influence over the president's social media histrionics.
"I say you have to let Trump be Trump. That is what has made him successful over the last 30 years. That is what the American people voted for," Lewandowski said. "And anybody who thinks they're going to change Donald Trump doesn't know Donald Trump."
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Kelly's start follows a tumultuous week, marked by a profane tirade from the new communications director, Trump's continued attacks on his attorney general and the failed effort by Senate Republicans to overhaul the nation's health care law.
The United States flew two supersonic bombers over the Korean Peninsula on Sunday in a show of force against North Korea, following the country's latest intercontinental ballistic missile test.
The US also said it conducted a successful test of a missile defense system located in Alaska.
Sen Dianne Feinstein, D-Calif, said on CBS' "Face the Nation" that she hopes Kelly can "be effective," and "begin some very serious negotiation with the North and stop this program."
Another diplomatic fissure opened Sunday when Russian President Vladimir Putin said the US would have to cut its embassy and consulate staff in Russia by several hundred under new sanctions from Moscow. In a television interview, Putin indicated the cutback was retaliation for new sanctions in a bill passed by Congress and sent to Trump.
While Trump is trying to refresh his team, he signaled that he does not want to give up the fight on health care. On Twitter yesterday, he said: "Don't give up Republican Senators, the World is watching: Repeal & Replace."
The protracted health care fight has slowed Trump's other policy goals, including a tax overhaul and infrastructure investment. But Trump aides made clear that the president still wanted to see action on health care. White House budget director Mick Mulvaney said on CNN's "State of the Union," that senators "need to stay, they need to work, they need to pass something."