WASHINGTON — Rarely has a president taken office so focused on undoing his predecessor’s works as Donald Trump. Six months in, he has little to show.

Monday brought twin blows. Not only did the Affordable Care Act survive another Republican repeal effort, maintaining President Barack Obama’s signature domestic achievement, but Trump was forced to certify that Iran continues to comply with the nuclear deal that was the biggest foreign policy accomplishment of Obama’s second term.

Beyond those headlines, Obama’s Deferred Action for Child Arrivals program to shield some 750,000 recipients from deportation continues intact, much to the frustration of some of Trump’s most ardent backers. The tax hikes on upper-income earners, among the hardest-won battles of Obama’s first term, remain in effect. U.S. relations with Cuba remain open, following Obama’s normalization policy, despite Trump’s public show last month of tightening some travel and trade restrictions.

None of that means Trump has failed.

Halfway through his first year, Trump has achieved some of his goals, although his repeated boast that he has “signed more bills — and I’m talking about through the legislature — than any president, ever,” is untrue no matter how one counts.

His announcement that the U.S. will withdraw from the Paris climate accord in 2020 has been the best-known part of a concerted administration effort to roll back Obama-era environmental initiatives.

And even before his election, Trump’s campaign against trade agreements roused opposition that helped kill Obama’s proposed 12-nation Pacific trade pact and slow the expansion of global trade deals. Trump formally withdrew the U.S. from the by-then-moribund Trans-Pacific Partnership on his first workday after his inauguration.

But Trump’s concentration on repealing what his predecessor did, rather than putting forward initiatives of his own, has also hampered his effectiveness to a remarkable degree.

One of the truisms of American government — as Trump now is learning to his dismay — is that taking away a benefit is generally harder than starting something new.

That’s one reason why presidents typically prefer to push their own agendas, rather than focus to the extent Trump has on uprooting their rival-party predecessors’ actions.

Presidents Dwight Eisenhower and Richard Nixon did not, for example, undo the New Deal or Great Society programs of, respectively, Franklin D. Roosevelt and Lyndon Johnson.

Obama’s opening year centered on efforts to recover from an economic crisis, re-regulate Wall Street and pass what became the Affordable Care Act.

Trump’s campaign did feature elements of what could form the basis of a distinctive first-year agenda. He talked about a massive plan to rebuild the nation’s roads, bridges and airports. He called for wholesale renegotiation of trade agreements. He advocated a sharp reduction in the number of legal immigrants allowed into the country. He endorsed a plan to give tax money to families to pay tuition for private or religious schools. And he touted a rewrite of the tax code, along with deep tax cuts.

He has not delivered concrete proposals on any of those ideas.

In part, that failure to follow through on those ideas stems from the reality that each of them, with the possible exception of tax reform, divides the GOP. Some of Trump’s proposals, such as those on trade, divide his own administration. Whether Trump’s ideas on taxes fit those of his party remains unknown because he has never specified what he has in mind.

Compounding the problem of a divided party is Trump’s lack of interest in developing policy and his slowness in choosing people for top government jobs. Together, those deficits have left his administration hamstrung in efforts to define an agenda of its own. That’s left Trump with the agenda the Republicans developed during the Obama years — one built around opposition to the party then in the White House.

The administration’s weakness on policy advocacy has been glaring over the last week as the effort to repeal and replace Obamacare dangled in the Senate. Trump made no concerted effort to win over public opinion — no extended speeches laying out a case for the Republican bill, no news conference to answer questions about his position.

On his preferred method of communication, Twitter, Trump sent more than 60 messages in the week leading up to the collapse of the GOP Obamacare repeal effort Monday night. Only six concerned health care.

When all was over, Trump’s response Tuesday was to express frustration.

“Let Obamacare fail,” he said at the White House. “We’re not going to own it. I’m not going to own it. I can tell you, the Republicans are not going to own it.”

As Trump staffs up government departments and regulatory agencies, his administration should be in stronger position to build an agenda around initiatives he’ll be more willing to own. But so long as he remains more focused on undoing Obama’s work than building something of his own, that cry of frustration is unlikely to be his last.

david.lauter@latimes.com