By NANCY BENAC, Associated Press

WASHINGTON (AP) — President Barack Obama had a "pen and phone" strategy. President Donald Trump has an eraser.

Since his first days in office, Trump has set out on a wholesale reversal of a long list of accomplishments that Obama achieved through executive action — a less enduring means than the hard-and-fast language of legislation.

The latest Obama-era policy to fall is the program shielding from deportation hundreds of thousands of young people brought into the country illegally as children. The Trump administration on Tuesday said the government would stop issuing new work permits while lawmakers debate whether to pass another solution.

In explaining his decision, Trump accused Obama of making "an end-run around Congress" to protect the so-called "dreamers." In effect, this time it's Trump making an end-run around Obama.

Obama, coming out of semi-retirement, retorted that Trump's action was a "cruel" and "self-defeating" decision tinged with politics.

It was yet another demonstration of the easy-come, easy-go nature of presidential achievements attained through unilateral action: What one president does by executive fiat, the next can just as quickly overturn.

And it's not just a Trump-Obama dynamic. Trump's executive orders will be subject to revision by his successor. And Obama didn't hesitate to reverse the actions of his predecessor, George W. Bush.

For all of that, though, Trump has been "unusually aggressive in his use of unilateral powers," says Kenneth Mayer, a University of Wisconsin-Madison political scientist and expert on presidential powers and executive orders.

While it's hard to systematically rank presidents on their use of executive actions, Mayer says "there are examples of Trump going beyond what other presidents have done in terms of the frequency and nature of unilateral action."

Trump, lacking any major legislative accomplishments despite the advantage of a Republican-controlled Congress, has issued dozens of executive orders and actions during the past seven months that have had a sweeping effect across the scope of government. They range from huge shifts in international policy to minor tinkering with obscure federal regulations.

He's pulling the United States out of the landmark Paris climate agreement through which nearly 200 countries had committed to combat global warming by reducing polluting emissions. He's scrapped an Obama administration policy that let national parks ban the sale of bottled water to fight littering. His Education Department has lifted Obama-era guidance to schools to allow transgender students to use the bathroom of their choice. He's ordered up two deregulatory actions for every new regulation issued. He tweeted out word that transgender individuals would no longer be allowed to serve openly in the military, as provided by the Obama administration, forcing the Pentagon to scramble to draft new rules to that effect.

Trump's actions on environmental matters extend well beyond climate change: He's moved to rip up Obama's Clean Power Plan, regulations that sought to reduce emissions from coal-fired power plants. His executive order on regulatory reform has been cited by EPA Administrator Scott Pruitt as a reason to delay or roll back a raft of Obama-era environmental regulations, from cleaning up water pollution from coal mines to blunting limits on emissions of toxic mercury from power plant smokestacks.

There's likely plenty more to come: Trump's Labor Department wants to undo an Obama administration rule extending mandatory overtime pay to 4.2 million more workers. And the administration is reviewing a potential rule that would let employers opt out of providing no-cost birth control to women for religious and moral reasons.

Some changes have been harder to impose than Trump expected: His initial attempt at an executive order temporarily banning travel to the U.S. from several Muslim-majority countries and suspending the U.S. refugee program hit roadblocks in the courts. On his second attempt, the Supreme Court allowed only a sharply scaled back version of the order to go forward pending arguments scheduled for October. Despite his Jan. 25 executive order to jumpstart construction on a U.S.-Mexico border wall, the structure is still far from reality.

Obama has largely bitten his tongue as Trump rolled back policy after policy from his presidency. But the former president spoke out clearly Tuesday about the "dreamers" program, saying in a statement that his order had been based on "the well-established legal principle of prosecutorial discretion, deployed by Democratic and Republican presidents alike." He said he acted unilaterally only after Congress failed to send him legislation to protect the "dreamers."

Obama stepped up his use of executive actions in 2014 as he became frustrated with how difficult it was to push legislation through Congress.

He famously declared: "We're not just going to be waiting for legislation in order to make sure that we're providing Americans the kind of help they need. I've got a pen, and I've got a phone."

But the more enduring nature of legislation vs. flimsier executive actions is clear in the difficulty that Republicans have had in repealing Obama's health care plan: It barely squeaked into law in 2010 but Republicans have been unable to vote it out after pledging for years to repeal it.

Presidents know their executive orders can be revoked with the stroke of a pen by their successors. They also know they can put the next president in a bind by creating a program that will be politically difficult to rescind. Trump agonized over his "dreamers" decision and caught criticism for trying to navigate a middle ground by proposing to gradually phase out the program while inviting Congress to come up with a permanent fix.

White House press secretary Sarah Huckabee Sanders was left to insist: "It's not cold-hearted for the president to uphold the law."

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Associated Press writers Michael Biesecker, Laurie Kellman and Maria Danilova contributed to this report.

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Follow Nancy Benac on Twitter at http://twitter.com/nbenac