On Wednesday afternoon, after this story was published, President Trump issued an order grounding all Boeing 737 MAX airplanes in the United States. 

In the wake of the second crash of a Boeing 737 MAX 8 passenger jet in the past five months, a number of countries have grounded the plane until an investigation is complete. 

But the United States stands nearly alone in allowing the plane to continue to fly, and the government took some aggressive steps Tuesday to make clear it has no intention of following other countries' leads.

The only thing is, it might not be up to the government.

There's a lot to recap from the past day or so, but ultimately, this might be most important development: The unions representing flight attendants and other employees at three of the four major U.S. airlines all took a stand Tuesday asking the government to ground the jet.

And it's worth remembering it was exactly this type of pressure from flight attendants that has been credited with helping bring the shutdown to a head in January -- after the head of the union representing United flight attendants called for a general strike in the U.S. if it didn't end.

Among other developments Tuesday:

  • The European Union joined China, Indonesia, Singapore, and Australia, along with many other countries, in grounding the planes.
  • Reports emerged that airline pilots "repeatedly voiced safety concerns about the Boeing 737 MAX 8 to federal authorities" in the months before Sunday's crash.
  • The U.S. Federal Aviation Administration said late Tuesday it has no plans to ground the planes. "Thus far, our review shows no systemic performance issues and provides no basis to order grounding the aircraft. Nor have other civil aviation authorities provided data to us that would warrant action," the agency's acting administrator said.
  • Perhaps to underscore this government stance, U.S. Secretary of Transportation Elaine Chao and her staff flew on a Southwest 737 MAX 8 from Austin to Washington, D.C., on Tuesday.

But the weather vane to keep an eye on is what the airline employees do now. Because if employees say they don't have faith in the planes, it will be almost impossible to keep passengers flying on them.

In fact, anecdotal reports have started to emerge of ordinary U.S. passengers who refuse to fly on MAX 8's. And at Southwest, the airline has begun waiving change fees for passengers who balk after realizing they're scheduled to fly on a MAX 8.

Here's the situation at the three largest U.S. airlines that fly 737 MAX 8's. (Delta is the one you'll notice missing from this list; that's because it doesn't fly this plane.)

Southwest Airlines

Southwest has 34 MAX 8's (out of 750 total Boeing 737's). It's still flying them, but as noted, will waive change fees for passengers who don't feel safe on them.

"It is absolutely absurd that the FAA won't ground the flights until at least they get the voice and data recordings back from this latest incident," John Samuelsen, head of the Transport Workers Union of America, representing 15,000 Southwest flight attendants plus 13,400 other workers, told CNBC.

Meanwhile, the union wrote to CEO Gary Kelly asking that the Boeing 737 MAX be grounded "until the results of the investigation into the voice and data recorders is completed and the cause of the catastrophe is determined."

American Airlines

"We continue to operate our 24 MAX 8 aircraft," American told Travel + Leisure.

But its flight attendant union wishes it wouldn't: "We are calling on our CEO, Doug Parker, to strongly consider grounding these planes until a thorough investigation can be performed," union president Lori Bassani said in a statement.

United Airlines

United doesn't fly any MAX 8's, but it does apparently fly the newer MAX 9.

"The United States has the safest aviation system in the world, but Americans are looking for leadership in this time of uncertainty," said Sara Nelson, president of the union representing United flight attendants.

Will any of it matter? Well, it was only a few weeks ago that Nelson was named the "most powerful flight attendant" in America for her role in helping bring about the end of the government shutdown.

And in an age of activism and instant communication, these employees are in a position to take the lead and influence issues like this in a way they never were before.