With ambitious plans for his first 100 days in office, President-elect Trump has shown full determination to grab the helm from President Obama and steer the country on a very different course.

Throughout his campaign, the Republican candidate promised to undo major pieces of the Obama administration’s domestic and foreign policy achievements, from repealing most of Obamacare and scrapping recent gun control rules to undoing major immigration reforms and building a new wall on the border with Mexico. He reiterated these intentions in his Contract with the American Voter, a plan released in October charting the first 100 days of his administration.

But what can Trump actually do with the stroke of a pen, and where might Congress -- or the Constitution -- stand in his way?

Obama was recently asked in an interview with the New Yorker magazine if he thought his accomplishments over the last eight years would be out the window with Trump in the White House.

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"I think that the possibility of everything being out the window exists," he said. "But, as a practical matter, what I’ve been saying to people, including my own staff, is that the federal government is an aircraft carrier, it’s not a speedboat."

Obama, however, does have good reason to be mightily concerned about his legacy. For one, Trump will be working with a Republican-controlled House and Senate whose leadership is generally in step with many of his goals and eager to overturn laws like the Affordable Care Act (Obamacare) and scrap any number of environmental regulations.

Moreover, some of Obama's most notable achievements were made through executive action, many of which Trump will have the power to undo almost immediately after taking office.

Executive orders and actions explained

The president heads the executive branch of government, and as such, is technically supposed to be enforcing laws, not making them. As we all "learned" (or were supposed to learn) in high school government class, that's the role of the legislative branch. The president does have the authority to veto legislation passed by Congress (the veto can be overridden with a two-thirds vote in both houses). However, he lacks the power to repeal laws that have already been enacted.

But there are some other legal options in the presidential bag of tricks.

That's where the power of executive action comes into play. The president can take these actions to create new rules that often have the full force of law and don't require congressional approval. And that includes the power to quickly reverse executive actions taken by a previous administration.

"Executive action" is a general term referring to a broad range of presidential directives, some of which have more legal heft than others. These include technical-sounding things like executive orders, memorandums, proclamations, and proposals. (To clarify, executive orders are a type of executive action, and they shouldn't be referred to interchangeably.) If you want to get more into the nitty-gritty, the Congressional Research Service provides a good explanation of the differences.

Every president, from George Washington to Obama, has used executive power, collectively issuing more than 13,000 executive actions according to one count. The text of every executive order from 1937 through August 2016 can be found here.

Some of the most consequential and controversial executive actions in history include President Abraham Lincoln's suspension of the writ of habeus corpus and the Emancipation Proclamation during the Civil War, President Franklin Roosevelt's order that led to the Japanese-American internment camps during World War II and President Harry Truman's 1948 order to integrate the armed forces. Incidentally, Roosevelt issued far more executive actions than any other president in history.

Obama's endangered actions

Since 2014, Obama has relied frequently on executive power to bypass a Republican-controlled Congress that was determined to stymie his agenda.

Obama told reporters in January 2014: “We are not just going to be waiting for legislation. I’ve got a pen and I’ve got a phone, and I can use that pen to sign executive orders and take executive actions and administrative actions.”

His actions (most of which were not actually executive orders, but rather memorandums and proclamations) have had sweeping impact (see the full list of his presidential actions here). They include a 2012 measure to protect hundreds of thousands of undocumented children from deportation (Deferred Action for Childhood Arrivals), as well as a 2014 expansion of this action meant to protect millions more young people and parents. This later order was later halted by a federal court, ruling the expansion unconstitutional.

Obama also single-handedly ordered a dramatic reduction in the nation's carbon emissions (an action known as the Clean Power Plan, that also remains tied up in federal court), placed limits on various forms of student-loan payments and tightened gun sale regulations. In December 2014, he even re-established diplomatic relations with Cuba. All without consent from Congress.

Critics of Obama's executive actions argue that it is a blatant overreach of his power and an unconstitutional attempt to bypass the legislative branch. Former House Speaker John Boehner (R-Ohio) twice sued the administration, accusing the president of using a "king-like authority at the expense of the American people and their elected legislators."

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But while executive action is a powerful tool, it can also result in a fragile, potentially short-lived outcome. Unlike laws enacted by Congress, these actions can be wiped away, literally overnight, by a successor with opposing views (enter President Trump). What one president enacts, another can undo.

When Obama took office in 2009, for instance, he immediately restored federal funding for international groups that perform abortions. His Republican predecessor, George W. Bush, had taken that funding away, which had earlier been brought back by Bill Clinton, a Democrat, after it had been first withheld by Ronald Reagan, a Republican. Ah partisanship!

In his October "contract," Trump pledged to "cancel every unconstitutional executive action, memorandum and order issued by President Obama."

For a complete list of Trump's major priorities during his first days as president, see this excellent New York Times graphic: "How Hard (or Easy) It Will Be for Trump to Fulfill His 100-Day Plan" as well as this Lowdown interactive post tracking nine major issues.

As the Times notes, Trump will instantly have tremendous leverage to upend many of Obama's accomplishments with the stroke of a pen. Some of his predecessor's major executive actions are on the chopping block, including DACA. Trump has also vowed to, among other things, immediately get rid of Obama's gun control actions, declare China a currency manipulator and withdraw the U.S. from both the Trans-Pacific Partnership trade deal and the Paris climate agreement.

Not all of Trump's pledges, however, are as simple or easy to push through as he's made them seem. Many of the president-elect's proposed changes will likely be subject to lengthy bureaucratic hurdles, constitutional legal challenges and, in some cases, congressional approval. Thia includes his promise to scrap Obama's environmental rules, which could get entangled in a lengthy bureaucratic and legal process (unless Congress took it on directly).

Trump also pledged to "begin removing the more than 2 to 3 million criminal illegal immigrants." Carrying out these deportations quickly, the Times notes, would likely violate due process and require additional funding from Congress. It's also unclear if Trump would have the authority to "cancel all funding to sanctuary cities," as he's threatened to do, without congressional approval.

And then there are the pledges Trump has made that require congressional approval. These include repealing and replacing Obamacare (an actual law, not an executive order), funding the construction of a wall along the border with Mexico and making sweeping tax cuts.

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And pushing all that and more through Congress "within the first 100 days," even with a supportive Republican Congress, will be a tremendous undertaking.

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The Power of Executive Action: What Trump Can Actually Do in His First 100 Days (with Lesson Plan)