A primer on executive power: What Trump can and can't do

WASHINGTON — Starting Tuesday night, as Donald Trump's victory in the presidential election started coming into focus, questions flooded social media:

Could Trump overturn Roe v. Wade, the Supreme Court decision that legalized abortion nationwide?

Could he abolish same-sex marriages?

Could he increase deportations?

[Trump wants to 'drain the swamp,' but change will be complex and costly]

How about global warming? Could Trump pull out of U.S. agreements on climate change?

What about "Obamacare"?

The chief executive has a lot of authority over the government, as President Barack Obama has demonstrated. But as Obama has found, he also faces tremendous constraints from Congress, the courts and existing laws.

Here are some areas in which President Trump could act quickly and where he could not.

Could Trump overturn Roe v. Wade?

Not on his own. The Supreme Court decision in 1973 struck down state laws around the country that banned abortions. Ever since, anti-abortion groups have called for reversing the ruling.

The decision could be overturned two ways — by a new Supreme Court decision or by a constitutional amendment. The high court has reaffirmed the Roe decision repeatedly, most recently by a 5-3 vote in June that struck down a Texas law that limited abortions.

In theory, if enough vacancies arise on the court, Trump could appoint a sufficient number of justices who would overturn Roe. In practice, the Supreme Court has had a majority of Republican-appointed justices since Roe was handed down, except for the past few months of a 4-4 tie, and that's never happened.

As for a constitutional amendment, the president has no role in that process. Amendments have to pass both houses of Congress by a two-thirds vote and be ratified by three-quarters of the states — a high bar.

What could Trump do about immigration?

A lot. The law gives the president broad authority over immigration policy. Trump could speed up deportations, revoke Obama's program that shielded so-called Dreamers — people who came to the U.S. illegally as children — and curb certain categories of visas for legal immigrants.

But as Obama found, there are limits. Courts struck down his effort to expand the deportation shield to cover millions of adults.

Trump also could not build a wall along the Mexican border without money from Congress, although he might be able to get started on construction by shifting funds already in the federal budget.

[Republicans in Congress plan swift action on ambitious agenda with Trump]

Could Trump ban Muslims from coming to the U.S.?

Legal experts have said he probably could: Noncitizens seeking to enter the country generally aren't protected by the Constitution's guarantee of equal protection of the laws.

But as of Thursday morning, mention of the Muslim ban had disappeared from Trump's campaign website as the president-elect tried to portray a less controversial, more presidential image.

Trump could limit the entry of refugees from Syria, Iraq and other war-torn Middle Eastern countries. He probably would face pressure from U.S. allies not to do so, but he could disregard that.

Are same-sex marriages at risk?

No. Just as with abortion, the president can't overturn a Supreme Court decision by himself, although he could appoint more conservative justices. Even then, the chance that the high court would reverse itself is very small. Unlike abortion, same-sex marriage has steadily gained in public approval.

Trump could try to reverse some Obama administration rulings that favored same-sex couples on issues such as federal benefits. He hasn't suggested that he would. And if he did, his decisions could be challenged in court.

Could Trump reverse current policies on climate change?

He could and quite probably will.

Trump has promised to withdraw the U.S. from an international agreement made in Paris last year that called on countries to reduce emissions of carbon dioxide and other greenhouse gases. And he has said he would scrap Obama administration rules that aim to shift electricity generation away from coal and oil toward cleaner sources.

Since those policies were adopted by the Environmental Protection Agency, using its authority under the Clean Air Act, they could be reversed by the EPA. But environmental groups could — and almost certainly would — go to court to try to block any such reversal. At minimum, the groups could tie up the Trump administration for a long time.

[Trump meets with Obama at the White House as whirlwind transition starts]

What happens if Trump tries to repeal Obamacare?

Trump has promised to ask Congress to repeal the Affordable Care Act and replace it with something else.

The repeal part is easy since the Republicans control both houses of Congress. The replace part is a lot harder.

Republicans have never been able to agree on a plan to replace the current law, and they would be under tremendous pressure not to simply repeal the law without a backup plan. Doing so would cause some 20 million people to lose health care coverage.

So carrying out Trump's pledge may take quite a while, and, in the end, the GOP may have to settle for less dramatic change than the rhetoric might imply.

Could Trump end federal funds for Planned Parenthood?

Congress would have to act to accomplish that goal, which many conservatives have sought because of the group's role in providing abortion services.

Whether a ban would pass both houses, especially the Senate, remains uncertain.

The federal money that goes to support Planned Parenthood's health care activities does not involve abortion but rather funds the group's other services, including mammograms and other preventive care for women. Several Republican senators have supported Planned Parenthood in the past because those services are popular in their states.

During the campaign, Trump threatened to appoint a special prosecutor to investigate Hillary Clinton. Could he?

The president, acting through the attorney general, has broad authority to appoint prosecutors. But appointing a prosecutor to go after a defeated campaign rival would be unprecedented, and Trump's aides already have started stepping away from that idea.

Could Trump order police to adopt "stop-and-frisk" policies?

No. During the campaign, Trump defended the use of so-called stop-and-frisk policing tactics. But police departments don't work for the federal government, they work for state and local governments.

In some cities, stop-and-frisk has been limited by court decisions. In other places, it has been dropped by police chiefs who decided it was not reducing crime enough to justify the tension it created in minority communities.

Trump talked about making libel law less protective of the press. Could he?

No. There is no federal libel law. Each state has its own law. A Supreme Court decision handed down more than 40 years ago — New York Times v. Sullivan — provides broad protection for the press in covering public figures.