Krugman's third paragraph completes his framing:
And there was a good reason they seemed so tongue-tied: Out there in the real world, none of the disasters their party predicted have actually come to pass. President Obama just keeps failing to fail. And that’s a big problem for the G.O.P. — even bigger than Donald Trump.

Well, maybe.  First let's point out several reasons why Trump is still a very big problem for Republicans:

1.  immigration:  his remarks on the subject, on Mexicans in particular, are not only making it almost impossible for Republicans to improve their performance among Hispanics, but they are also energizing them to participate. The folks at Latino Decisions have run the numbers and unless Republicans substantial improve their performance with this demographic they cannot win the Presidency. Oh, and as our own Markos has reminded us, 50,000 Hispanics become eligible to vote every month.
2. women - yes, some of Trump's remarks are totally obnoxious, and that's putting it mildly. But that only serves to highlight how bad the Republican field is on issues relating to women, all of them.
3. political corruption - Trump'r remarks about why he gives money and what he gets taps into an anger that is palpable, that the system is failing the people, and many people know it, which is also why Sanders is doing so well. Those remarks are a perfect set-up to Democratic attacks on role of the Koch Brothers and similar big bucks donors, including their attempt to designate the candidate.
Even with all that, and with other issues as well, however, Krugman is actually correct.  Increasing numbers of people are recognizing that for all its flaws, the Affordable Care Act has done them good. The news keeps reinforcing that point, for example, the word from its trustees that Medicare has slowed its acceleration of growth to the point where in fact it is pretty close to sustainable.  Do Republicans insist that it - and Social Security - are so broken that they have to be replaced?  In wanting to get rid of Medicare have they forgotten the tea=party types who now represent the heart of their base insisting in 2009 that they wanted to keep government hands off their Medicare?
Krugman's column provides some actual economic data to make his argument.  No surprise, considering his field of expertise.
As for the idea that Obamacare would be a job-killer, a point to which Marco Rubio has returned, Krugman points out that
in the year and a half since Obamacare went fully into effect, the U.S. economy has added an average of 237,000 private-sector jobs per month. That’s pretty good. In fact, it’s better than anything we’ve seen since the 1990s.

Since the 1990s.

The last time a Democrat was President.
In  contrast to the 8 years of Republican leadership under a man whose last name was Bush.
Krugman does not say that, but I thought it worth mentioning.
And speaking of Bush, Krugman reminds us that JEB! governed during a housing bubble in Florida, having the good fortune to leave office before it burst. Oh, and as for "St. Ronnie?"  A comparison of Obama's administration with his shows that Obama entered office with higher unemployment that that notable and that his unemployment rate now is lower than it was at the equivalent moment in Reagan's second term.
Of course, what Krugman does not say is that too many in the political press are far more focused on horse race info, like who has raised how much, who your staff is.  And of course, what the punditry considers gaffes, although I note that each time they declare Trump's candidacy over his poll numbers seem to go up.  Still, whoever will be the Democratic candidate will have plenty of opportunity to remind the American people of the achievements of the Obama administration as compared with any of the following

- Reagan
- GHW Bush
- GW Bush

and certainly when it comes to employment and economic development , some of the Republican candidates are particularly vulnerable - are you listening Scott Walker?
On energy, domestic oil production is up under Obama and imports are down. Although the energy bubble in North Dakota has seemingly deflated a good deal, and this administration has done imho a poor job asserting itself on fracking for natural gas, it is interesting to see that increasing number of Americans are beginning to understand we still have to change our ways, which may be why power producers are for the most part moving further away from coal (except perhaps for Dominion Power in Virginia) and coal companies are in increasing financial jeopardy.
Krugman sets up his final paragraph by asserting Obama would be a disaster in every direction.
Let's parse that final paragraph:
But he hasn’t. I’m not saying that America is in great shape, because it isn’t.

 There is much still to address, but think how horrible it would have been without ARRA, the stimulus, and without Obamacare.

Economic recovery has come too slowly, and is still incomplete;

- too many people have left the work force because they cannot find jobs, wages are still too low, but again, consider what could have been

Obamacare isn’t the system anyone would have designed from scratch;

- for my taste, and for many, there is still too much profit for insurers and Pharma, but they are now committed to the notion of expanded healthcare, and thus are not likely to be supportive of efforts to "repeal" that do not guarantee them the same revenue stream, and increasingly the American people are pleased with what they are getting

and we’re nowhere close to doing enough on climate change.

 We will still see major fights on the new rules the administration wants to promulgate, but so far it looks the like they are somewhat supported within the Courts. And when we look at other nations - Denmark now exporting electricity from wind, for example - the path we must go is inevitable if we want to continue to have both a growing economy and avoid destroying our agriculture and our coastal communities.

Krugman's final sentence is absolutely on point:  
But we’re doing far better than any of those guys in Cleveland will ever admit.

And if we listen to them, most of us would be royally screwed - economically, on climate, on health care, and most of all on important civil liberties.

Don't take my word for it.
Read the column.
Peace.