Susan Page

USA TODAY

WASHINGTON — Not so fast.

In his first three weeks in office, President Trump has launched a dizzying flurry of actions, dominating the headlines and sparking controversy. But to his frustration, he has begun running smack into constraints mandated by the Constitution and imposed by political reality.

Without acknowledging he's being forced to trim his sails, the president is moving to delay some campaign promises and downsize others, steps that sometimes have been overshadowed by a continued stream of the defiant tweets that marked his political rise.

A moratorium on immigrants from seven majority-Muslim nations? While insisting the administration will prevail over a federal judge's decision blocking implementation of the executive order he signed, the White House is drafting a narrower version designed to avoid some of the judicial objections. Confront China? In a phone call with Chinese President Xi Jinping late Thursday — and in a wake of a diplomatic firestorm over Trump's post-election conversation with Taiwan's leader — Trump explicitly endorsed the "one China" policy. Immediately repeal the Affordable Care Act? Now he says crafting a replacement, an issue that divides congressional Republicans, may extend into 2018.

To be sure, Trump has taken steps that have had immediate consequences, from pulling out of the Trans Pacific Partnership trade deal (though it hadn't taken effect yet) to clearing the way for the Keystone XL and Dakota Access pipelines.

But he's discovering how the Constitution's structure, federal laws and rival power centers — from state governments to federal bureaucrats to foreign capitals and the news media — make leadership in the Oval Office a more complicated calculation than in the corporate suite.

"He lived in a world where, by and large, he got his way, and when he didn't get his way, he would sue people or they would sue him," says Tom Cronin, a Colorado College political scientist and co-author of The Paradoxes of the American Presidency. Trump seems "annoyed" by the steep learning curve, Cronin said, and is discovering "He can't quite sue the Senate."

On CBS' Face the Nation Sunday, White House policy adviser Stephen Miller said the president was assessing his options on the immigration ban. "For one thing, we can take the case to the Supreme Court on the emergency stay; we can go back to the district court and we can have a hearing on the merits," he said. "Additionally, we're considering new and further executive actions that will enhance the security posture of the United States."

Trump is the first president in history never to have served in the government or the military before moving into the White House. That was a potential vulnerability he turned into an asset during the campaign, portraying himself as a no-nonsense business leader with the strength and skills to negotiate deals and manage the economy in ways that had eluded his presidential predecessors. "I alone can fix it," he declared in his speech to the Republican National Convention in July.

And he is hardly the first president who has been frustrated by the limits of his authority and the power of others. Last year, federal courts blocked an executive order signed by President Obama offering some protection to an estimated 4.3 million illegal immigrants who were parents of U.S. citizens or lawful residents. In 1998, the House of Representatives impeached President Clinton for perjury and obstruction of justice, though he was acquitted by the Senate.

But Trump's impatience, his combative persona and his preference for unilateral action — characteristics that may have served him well in the real estate business — have brought more confrontations with a sharper edge than other modern presidents in the early days of their tenure. He hasn't shown the traditional public deference for the co-equal branches of government, deriding a "so-called judge" who ruled against him in the immigration case and labeling Senate Democratic Leader Chuck Schumer as "head clown."

It's just possible that memories of that moniker may not be helpful when Trump needs to lobby Schumer for support on legislation.

"Being head of a family business is just about the worst preparation imaginable for the institutional constraints of Washington," says William Galston, a veteran of the Clinton White House staff. "Donald Trump is in for a set of experiences that will be entirely novel to him."

Read more:

The first 100 days of the Trump presidency

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Trump team assesses legal options on travel ban

So far, Trump hasn't moderated his rhetoric or tempered his tweets. On Saturday night, he bashed the news media. "I am so proud of my daughter Ivanka. To be abused and treated so badly by the media, and to still hold her head so high, is truly wonderful!" On Sunday morning, he turned his fire on billionaire Mark Cuban, a critic who endorsed Democrat Hillary Clinton for president last year. "I know Mark Cuban well," Trump wrote. "He backed me big-time but I wasn't interested in taking all of his calls. He's not smart enough to run for president!"

However, he has moderated some of his policy positions, from the timetable for repealing Obamacare to the specifics of the immigration ban. Trump told reporters he continues to believe that waterboarding was effective in interrogating terror suspects, but he said he would defer to Defense Secretary James Mattis, who opposes them. After meeting with Secretary of State Rex Tillerson last week, European Union leaders said they had been assured the United States would uphold the Iranian nuclear deal, which Trump denounced during the campaign.

The president's meeting Wednesday with Israeli Prime Minister Benjamin Netanyahu will test Trump's new, more skeptical stance toward expanding settlements in the occupied West Bank.

Charlie Black, a Republican consultant who has known Trump for years, predicts that some of the skills the president honed negotiating real-estate deals will end up working in the Oval Office as well. "He had a lot of wins and losses," including some that demanded patience in negotiations with banks. His strengths as a salesman could be used to persuade members of Congress, Black says: "He ran into barriers before. He'll have to adapt to them."