President Trump took aim this week at two of the biggest parts of his predecessor Barack Obama's legacy.

On Friday, Trump declined to certify the Iran nuclear deal as being in the national security interests of the U.S., though he stopped short of immediately withdrawing from the pact. The nuclear agreement is Obama's chief foreign policy accomplishment.

Previously on Thursday, Trump signed an executive order easing rules imposed by the Affordable Care Act, former President Obama's signature domestic policy achievement. He also ordered the federal government stop cost-sharing reduction payments to insurers under Obamacare.

Both moves followed Trump's decision to rescind after six months the Deferred Action for Childhood Arrivals program protecting some young illegal immigrants who arrived in the U.S. as minors. DACA was also created by an Obama executive action.

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Trump's Thursday executive order was designed to make it easier to sell cheaper health insurance plans under Obamacare. DACA and the cost-sharing reduction payments were both terminated because they were executive actions with no statutory basis that were being challenged in the courts.

But Trump's back-to-back moves against Obamacare and the Iran nuclear deal also signaled his intention to erase major parts of his predecessor's record. That is certainly how many alumni of the Obama administration see it.

"Trump is endangering the health care of Americans and constraints on Iran's nuclear program," tweeted former Obama foreign policy adviser Ben Rhodes. "Only organizing principle: opposition to Obama."

In none of these cases did Trump take the most radical steps possible, although he did act within the constraints of his executive authority. On Iran, cost-sharing reduction payments, and DACA, Congress will play a large role in determining what happens next. Republican lawmakers have failed to pass Trump-backed bills to partially repeal and replace Obamacare.

Some of these measures reflect Trump's negotiating posture: Create a deadline whereby a policy outcome no one wants will occur in order to force Democrats and Republicans in Congress to the bargaining table. Trump campaigned on being a master deal-maker, and he in particular described the Iran nuclear deal as the product of bad negotiators.

"We got weak inspections for no more than a short term and temporary delay in Iran's path to nuclear weapons," Trump said on Friday.

"The previous administration lifted sanctions just before what would have been total collapse of Iranian regime," he added.

Trump also exhorted the Democrats to deal with him on healthcare.

"Massive subsidy payments to their pet insurance companies has stopped," he said in a tweet about "imploding" Obamacare. "Dems should call me to fix!"

It is also the case that many Obama-era initiatives were vulnerable to reversal by Trump because the 44th president relied so heavily on executive action, especially after the passage of Obamacare (which, as an act of Congress, has proved durable despite unified Republican control of Washington).

Nevertheless, Trump's political career began as a reaction against Obama. Trump questioned Obama's birthplace and eligibility for the presidency. Obama, in turn, often ridiculed Trump. It has even been speculated that Obama's jibes during his 2011 White House Correspondents' Dinner speech helped spur Trump to run for president.

Trump has, so far without evidence, accused Obama of questionable surveillance and inappropriate "unmasking" of his associates. Obama officials, and the former president himself, have remained highly involved in opposition to Trump's policies.

But right now, Trump has the power to actually contravene Obama's executive actions and agreements. This week, he has made use of it.