Donald Trump promised to undo the Iran nuclear deal hammered out by the Obama administration and its international partners with Tehran.

He promised to take the United States out of the Paris climate agreement signed earlier this year after years of international negotiations.

He promised to repeal the Affordable Care Act, President Obama's health care reform, better known as "Obamacare."

He vowed to undo numerous trade agreements negotiated by previous US administrations with various nations and replace them with better deals.    

These are just four out of the range of existing agreements and laws Donald Trump promised to repeal upon becoming president. And with both chambers of Congress in Republican hands, what could hold a President Trump back from making good on these promises which played a key role in his change-driven campaign agenda?

But in Washington's political reality, the business of enacting - or repealing - legislation is often a little more complicated than they seem as a candidate on the campaign trail in Iowa or Florida.

Executive orders to ditch Iran and climate deal

Still, President Trump could pull the United States out of both the Paris climate agreement and the Iran nuclear deal. Since both agreements are not binding treaties, which would have required Senate approval President Obama was unlikely to get, but executive agreements, Trump could potentially issue an executive order repealing Obama's existing executive agreements.

But political experts are not convinced that a President Trump, when he takes office, will actually follow the path laid out by candidate Trump.

"It's unclear how that will play out," said David Canon, a political scientist with a focus on US Congress at the University of Wisconsin-Madison.

Since a US withdrawal from both of these agreements would have tremendous international consequences and effectively render both deals largely meaningless, such a move would be highly risky, said Canon.

"Such reckless, precipitous actions would cause too much disruption," said John Johannes, a congressional scholar at Villanova University. Trump has little understanding of policy and political process, noted Johannes, but the stable of advisors he will have at his side once he takes office "will understand politics in Washington a whole lot better than he does - at least I hope they do."

Repealing easier than replacing

Similarly with Obama's health care reform, said Canon, "the repeal part is easy, the replace part is much harder." But while revoking the Affordable Care Act, loathed by conservatives, would fulfill a long-standing Republican promise and hand them an important symbolic victory, it would also strongly impact those covered by it.            

"It seems unlikely they would simply remove health insurance from 20 million Americans without something to replace it," said Canon. The problem, he added, is that neither Trump nor the GOP has a prepared an agreed-upon replacement plan that could replace Obamacare.   

"It would be a national disaster to pull them off suddenly," said Johannes of the people use rely on Obama's signature health care program. "Republicans will have to be a lot more cautious than Trump has been saying."

While ditching the Iran deal, the Paris climate agreement and Obamacare would be possible and politically popular with most Republicans, that isn't the case for another central plank of Trump's campaign - getting rid of existing trade agreements and replacing them with US-friendlier deals.

Resistance on free trade stance

"He will run into resistance within his own party on that," said Canon. While some in the Republican Party have supported Trump's anti-free trade stance, many have not, as free trade has traditionally been a cornerstone of Republican orthodoxy.

What's more, since Trump would need Congressional backing to undo trade agreements like NAFTA he would likely face opposition from House Speaker Paul Ryan, a Republican free trade supporter. Ryan, however, is up for election in January, and some Trump supporters in Congress have already said they want to oust him. Ryan publicly refused to campaign with the GOP nominee after the release of an audiotape in which Trump bragged about getting away with groping women. But a replacement who could hold the party together better than Ryan is hard to come by, leaving Trump and his supporters to think twice before showing the lawmaker th door.   

Immigration promise

Even his promise to swiftly deport undocumented immigrants - a key rallying cry during the campaign - could be hard for Trump to fulfill.

"If he were really to follow through on his plan to deport 16 million illegal immigrants, he would run into real resistance from the Republican party because the backbone of the Republican Party is still small business people," said Canon. While Trump had already backtracked on this promise by vowing to begin deportations of those with a criminal record, it may not happen as quickly as some of his supporters expect.  

Unpredictable Trump

"I think he will be a little more cautious on that," said Johannes. "The last thing he needs is a big explosion in his first weeks in office."

For David Canon, President-elect Trump's victory speech could be an indication that President Trump may be more restrained than candidate Trump was. That's because in his initial remarks, said Canon, Trump focused on rebuilding infrastructure, rebuilding inner cities and support for veterans, but didn't mention any of the more divisive issues.

While there are clear political obstacles to fulfilling many of Trump's central campaign promises, if Donald Trump has proven anything during his campaign, then it is that he is a master in overcoming obstacles. He has also proven that, in the end, he, not his advisors, makes the important decisions.

"The trouble with Trump is nobody really knows him," Johannes said, "and the guy is reckless."