Obamacare is in deep trouble. The exchanges created by Barack Obama's signature achievement are attracting millions fewer customers than predicted. The makeup of the system's risk pool is older, sicker and costlier than anticipated. Some of the largest insurers in the nation have pulled out of the exchanges. Premiums are going up and expected to jump right before November's election. Perhaps more burdensome, deductibles have soared so high that some Americans who purchased coverage through the system are essentially self-financing their care.

Politically, Democrats own Obamacare, lock, stock and barrel. That includes Hillary Clinton, who is running on a pledge to "defend and expand the Affordable Care Act."

So far, the best solutions Democratic wonks and politicians have come up with to fix the failing system is 1) for the government to pump more taxpayer dollars into Obamacare, raising subsidies to reduce the hit the lowest-income customers take from rising costs; 2) for the government to enact more coercive measures to force Americans to buy insurance products they so far have declined to buy; and 3) for the government to institute a public option, a measure rejected by the Democrats who passed Obamacare in part because it was seen as a step toward a single-payer system.

Democrats counter that Obamacare has significantly reduced the number of uninsured in the U.S., but the bulk of that reduction has been due to the program's expansion of Medicaid, which could have been done without messing up the larger insurance system. (Also, studies indicate that Medicaid, while it gives those covered some peace of mind, doesn't seem to make them healthier.)

These days, for a large part of the non-Medicaid population buying insurance on the individual market, Obamacare has become a very troubling presence. There could hardly be a better issue for a Republican presidential candidate to use against the Democrat seeking to succeed Barack Obama.

Yet Donald Trump remains virtually silent on Obamacare. Look at Trump's last 10 speeches — the number since Trump began delivering prepared-text teleprompter remarks. All came during a period of bad news about Obamacare. But, according to the texts released by the campaign, one Trump speech didn't mention Obamacare at all, while several others devoted just a few — really, a few — words to the subject.

Starting with Trump's most recent speech, Saturday at Sen. Joni Ernst's "Roast and Ride" event in Iowa, he mentioned Obamacare once — in the context of immigration, not health care. "Hillary Clinton's immigration proposal will also give illegal immigrants access to Obamacare, Social Security, Medicare and all U.S. welfare," Trump said. That was it.

In Trump's speech last Thursday in Manchester, N.H., he mentioned Obamacare as part of a laundry list of pledges to create jobs, fix trade deals, cut taxes, cut regulations, and: "We will repeal and replace job-killing Obamacare." Then it was on to something else.

The day before, Aug. 24 in Jackson, Miss., Trump did the same thing: "We are going to cut taxes, reduce regulations, fix our trade deals, unleash American energy, and repeal and replace Obamacare." Nothing more.

The same day, Aug. 24, in Tampa, Trump was expansive, at least for him: "Hillary Clinton wants to expand job-killing Obamacare and put government in charge of healthcare," he said. "We are going to repeal and replace Obamacare, and create choice and competition that puts patients first."

On Aug. 23, in Austin, Texas, Trump very briefly vowed to repeal and replace Obamacare and to deny coverage to illegal immigrants.

On Aug. 22, in Akron, Ohio, he quickly promised to get rid of Obamacare, "saving another two million jobs over the next decade."

On Aug. 20, in Fredericksburg, Va., he didn't mention Obamacare at all.

On Aug. 19, in Dimondale, Mich., he said, "Obamacare will be repealed and replaced."

Remember that these are speeches that, even in the slimmed-down New Trump version, go at least 40 minutes. Five seconds is not a long time to devote to one of the biggest issues of the election.

On Aug. 18, Trump delivered his well-received "regret" speech in Charlotte, N.C. He also spent (for him) a relatively long time talking about Obamacare:

On healthcare, we are going to repeal and replace Obamacare. Countless Americans have been forced into part-time jobs, premiums are about to jump by double-digits yet again, and just this week Aetna announced it is pulling out of the exchanges in North Carolina. We are going to replace this disaster with reforms that give you choice and freedom and control in healthcare – at a much lower cost.

Finally, on Aug. 16, at his law-and-order speech in West Bend, Wis., Trump again waxed eloquent, at least for him:

On healthcare, we are going to get rid of Obamacare – which has caused soaring double-digit premium increases – and give choice to patients and consumers. Aetna, just today, announced they are dropping out – as are many of the major insurance companies.

Trump also hasn't made much of Obamacare in his press releases. In the last few months, he has dramatically stepped up his rapid-response efforts, sending out quite a few releases on the news of the day. On Obamacare, he had sent out all of one release in August, until he sent another out Sunday night ("Trump campaign statement on latest Obamacare failure").

In virtually all of the speeches included here, Trump has devoted a good deal of time to a quixotic mission to appeal to black voters. He has clumsily searched for a way to re-work the deportation part of his immigration policy. And, as usual, he has spent a lot of time doing TV interviews about all sorts of things.

And yet on an absolutely critical issue, one that cuts across health care and economic concerns, and one in which Trump has solid ground on which to attack Clinton, the GOP presidential candidate has spent virtually no time attacking a glaring Democratic vulnerability.

At one point, Republicans made opposition to Obamacare the centerpiece of their attacks on Democrats. Now that the system is actually coming apart, the GOP presidential nominee says virtually nothing at all.