"Running for Senate, I was 100% focused on our campaign, winning that race, and then serving the next six years in the United States Senate, O'Rourke said of his close-but-no-cigar challenge to Republican Sen. Ted Cruz earlier this month. "That was 100% of our focus. Now that that is no longer possible, you know, we're thinking through a number of things. Amy and I made a decision not to rule anything out."

That's a clear change from O'Rourke's insistence during the course of his 2018 campaign that he had no interest in running for president in 2020. "I will not be a candidate for president in 2020," O'Rourke said the day before the Texas Senate election. "That's, I think, as definitive as those sentences get."

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Here's the thing: O'Rourke should absolutely, 100% run for the Democratic presidential nomination in two years' time.

Why? Ask Cinderella.

"Opportunity is not a lengthy visitor," the princess said in " Into the Woods ," the mid-1980s Stephen Sondheim fairy-tale themed musical.

(Sidebar: That same "opportunity" quote is featured in Houston Rockets general manager Daryl Morey's Twitter bio .)

The point of the quote is simple: Moments that matter in your life don't stick around forever. You need to seize them or run the risk of missing them.

Which brings me back to O'Rourke. He started the 2018 election as a relatively unknown member of Congress from Texas who was tilting at the political windmill that is Ted Cruz because, well, no one else would. He ended it as a national Democratic rock star. He raised more than $70(!) million for his race. Polls showed him surprisingly competitive with Cruz.

And then, yes, he lost. And no, losing Senate candidates are not usually at the front of the line to run for president.

But O'Rourke's loss was narrow -- 200,000 votes out of more than 8 million cast -- and that slender defeat has done little to quell the heat he is producing among Democratic activists. The amount of energy and money O'Rourke was able to generate during the 2018 campaign has, without question, created an opportunity.

And it's an opportunity that O'Rourke may never get again.

Let's go through it. In January, O'Rourke will be out of a job. (He retired from his House seat to run for Senate.) His next opportunity to run for a major statewide office would be in 2020 against Sen. John Cornyn (R). Cornyn is a far less divisive politician than Cruz and will likely benefit from presidential-year turnout in Texas. If O'Rourke passes on that race, his next chance would be in 2022, when Gov. Greg Abbott (R) would be up for a third term. Abbott could be moving toward a presidential race in 2024 at that point -- so it's possible that the governor's race would be open. But even an open-seat race isn't any sure thing for O'Rourke -- or any Democrat -- in the state of Texas.

O'Rourke's best chance -- and it's not even that great a chance -- to win a major statewide office would be in 2022. And if he passes on this presidential race, his next opportunity to run for president wouldn't be until 2024. And if a Democrat beats President Donald Trump, then O'Rourke would have to wait until 2028(!!!) to run for president.

Now, O'Rourke is a young guy. (He's 46.) But politics is a fickle mistress. And the public has the attention span of a flea. Four years in politics is forever.

Think about this time four years ago. Republicans had just won another major midterm victory. Donald Trump's name wasn't even on the long list of potential GOP presidential candidates. Everyone assumed Hillary Clinton would be president. Honestly, looking back, it feels like 40 years ago, not just four.

Today, Beto O'Rourke is the hottest thing in Democratic politics. He's touched a nerve among Democrats in a way that evokes Barack Obama circa 2006-2007 . He's proven he can raise tens of millions of dollars to fund a national campaign.

In four or even eight years? There will be a new hottest thing -- and likely several. O'Rourke will be a guy who almost caught lightning in a bottle years back. Out of office, he won't have the platform to inject himself into the national debate. He will do that thing that politicians fear the most: Disappear (almost) completely.

If O'Rourke runs for president in 2020, he is, I think, a top-five contender for the nomination. There's no scenario -- either in Texas or nationally -- where he will have that sort of chance again.

Opportunity is not a lengthy visitor.