The Affordable Care Act is Barack Obama's signature achievement as president. It is the Democratic Party's most important legislative accomplishment in two generations. It is something Hillary Clinton, although she left the Senate by the time ACA became law in 2010, worked for all her adult life.

So why doesn't Clinton at least mention this great achievement as she campaigns for president?

A look at transcripts of Clinton stump speeches since she kicked off the general election campaign on Labor Day finds the Democratic candidate almost never talks about Obamacare. She doesn't promise to expand it. She doesn't promise to protect it. She doesn't extol its benefits. She just doesn't mention it.

There's no doubt Obamacare is in trouble. Enrollment in the exchanges has fallen far short of projections. The purchasers of policies have turned out to be older, and in need of more care, than expected. Major insurers are pulling out of the exchanges altogether. Premiums are going up. Deductibles are skyrocketing, meaning many are left to pay most of their healthcare costs themselves.

Here's one all-too-common headline, from Bloomberg on Friday: "Near 'Collapse,' Minnesota to Raise Obamacare Rates by Half." The situation is being repeated around the country.

On her website, Clinton proposes to "defend and expand the Affordable Care Act." She promises a public option, as well as a version of Medicare for all.

But she hasn't been saying that on the stump in the general election campaign. Instead, when Clinton speaks of healthcare, she speaks of making it affordable, as if there were not already a massive, coercive and far-reaching law called the Affordable Care Act. It's as if Obamacare never happened.

Clinton gave two speeches on Sept. 5, her Labor Day campaign kickoff. In the first, in Cleveland, she touted her role in "the fight for health reforms." She didn't mean Obamacare. Instead, she was talking about her work as first lady in the 1990s.

"I've spent my life fighting for kids and families," Clinton said. "During the fight for health reforms, some of you remember, powerful interests blocked our way, but I didn't give up. I turned around. I worked with Republicans and Democrats to help pass the Children's Health Insurance Program that covers eight million kids today." (CHIP was passed in 1997.)

Later, in Hampton, Ill., Clinton said much the same thing, touting her role in passing CHIP but saying nothing about Obamacare.

The next day, Sept. 6, Clinton spoke in Tampa, Fla. Again, she said nothing about the Affordable Care Act. She did, however, repeat a line she sometimes uses — that she wants to create "a healthcare system that works" — which could be interpreted as saying she doesn't think the present system, built by Obama and Democrats in Congress, is working.

On Sept. 8, Clinton traveled to Kansas City, Mo., to speak to the National Baptist Convention. She said nothing about Obamacare.

On the same day, Clinton spoke at the historically black Johnson C. Smith University in Charlotte, N.C. She pledged to "defend quality affordable healthcare for everybody, but we're going to get the cost down."

After time off for illness, Clinton spoke in Greensboro, N.C., on Sept. 15. She tied her healthcare remarks to her bout with pneumonia, and what she said didn't seem very positive toward Obamacare:

Lots of Americans still don't even have insurance, or they do but it's too expensive for them to actually use. So they toss back some Tylenols, they chug orange juice and they hope that the cough or the virus goes away on its own.

That wasn't exactly a ringing endorsement of the Affordable Care Act.

On Sept. 16, Clinton spoke to the Black Women's Agenda Annual Symposium in Washington. She said nothing about Obamacare.

That night, Clinton addressed the Congressional Hispanic Caucus Institute Public Policy Conference, also in Washington. She spoke after President Obama, and, with him in the room, apparently felt the need to mention his legacy a bit more than she does on the stump. Thus she made the only explicit mention of Affordable Care Act that I found in a month of speeches.

"Think of what we've achieved these last eight years," Clinton said, with Obama listening. "American businesses have created 15 million new jobs since the recession. Twenty million Americans have health coverage. And no one has seen a bigger drop in uninsured rates under the Affordable Care Act than Latino Americans."

On Sept. 17, Clinton spoke to the Congressional Black Caucus Foundation dinner in Washington, again sharing a bill with Obama. Her remarks were brief and included nothing about the Affordable Care Act.

On Sept. 19, Clinton spoke at Temple University in Philadelphia. She said a few words about CHIP, but nothing about Obamacare.

On Sept. 21, Clinton spoke in Orlando. Nothing on Obamacare.

After that, Clinton took a break for debate preparation. On the day after the debate, Sept. 27, she spoke in Raleigh, N.C. Nothing on Obamacare.

On Sept. 28, Clinton traveled to New Hampshire for a joint appearance with her former rival, Bernie Sanders. One of them spent some time discussing "a serious crisis in terms of primary healthcare." It wasn't Clinton.

On Sept. 29, Clinton spoke at an early-voting rally in Des Moines, Iowa. Nothing on Obamacare.

On Sept. 30, Clinton spoke in Ft. Pierce and also in Coral Springs, Fla. Nothing on Obamacare.

One month. Sixteen speeches. And exactly one mention of the Affordable Care Act — and that was because President Obama was in the room listening.

Maybe I missed a reference somewhere. Maybe Clinton plans to discuss Obamacare sometime in the future. But the point remains: When Clinton takes to the stump to campaign, her party's signature achievement is nowhere to be found.

At Clinton's highest-profile appearance in September — her debate with Donald Trump — moderator Lester Holt did not bring up Obamacare. Clinton could have mentioned it herself but did not.

Remarkably, Trump didn't bring it up either. That is surely frustrating to Republicans who hoped to make the Democrats' struggling and burdensome health care scheme a major issue in the debate, and the campaign as a whole.

But put aside Holt's and Trump's omissions for a more fundamental question. If Obamacare is so great, why isn't Hillary Clinton talking about it?