Rep. Tim Ryan (Ohio) just announced that he is pulling out of the race for the Democratic presidential nomination, shocking political observers who asked, “Wait, was he still running?”

I don’t want to be mean to Ryan, who seems like a sincere guy with worthy ideas. We interviewed him back in April, and he surprised us a bit by emphasizing the need for a Green New Deal to revive manufacturing. Though he’s known for having challenged then-House Democratic leader Nancy Pelosi (D-Calif.), Ryan is not as conservative as his reputation might indicate; ideologically, he’s right in the middle of the Democratic Party. And he wrote a book about mindfulness.

But alas, it was not to be. Ryan now joins the likes of Bill de Blasio, Kirsten Gillibrand, John Hickenlooper, Jay Inslee, Seth Moulton, Mike Gravel and Richard Ojeda on the list of people who ran for the Democratic nomination and then formally pulled out. There are still 18 candidates in the race; nine of them have qualified for the next debate, and a couple more may join them.

So what can we learn from Ryan’s demise, and that of the others who have departed? A few lessons:

You need a central rationale people can get behind. It’s remarkable how many candidates can’t come up with a simple answer to this question: “Why are you running for president?” Look, for instance, at the different fortunes of Sens. Elizabeth Warren (Mass.) and Kamala D. Harris (Calif.). They’re both smart, qualified and charismatic, even if in different ways (more on charisma in a moment). Yet, Warren’s campaign has been a huge success while Harris has struggled. It has more than a little to do with the fact that Warren has a clear articulation of her mission — fighting corporations and the wealthy to undo a corrupt system — while Harris can’t seem to explain in a succinct way what drives her candidacy.

It’s not a guarantee of success — Ryan wanted to help forgotten Americans, while Inslee, the governor of Washington, wanted to address climate change, both pretty simple ideas — but without it, you can’t win.

Qualifications and stature still matter. As much as the presidential campaign has opened up to anyone with some extra time and a dream, it’s still difficult to get taken seriously as a presidential candidate if you haven’t held one of a small number of jobs. The current leading candidates in the primary are a former vice president and two senators. Ryan — like former congressmen John Delaney and Beto O’Rourke, as well as Rep. Tulsi Gabbard (Hawaii) — was trying to go directly from an office in the House of Representatives to the White House, something that hasn’t been done since James Garfield did it in 1880.

That doesn’t mean there aren’t exceptions — South Bend, Ind., Mayor Pete Buttigieg is running a pretty strong campaign despite being mayor of a town of a little more than 100,000, and there’s also the guy currently in the Oval Office — but something about being in the House just makes a candidate look a bit small.

There’s no substitute for charisma. Look back over the last half-century or so of presidential elections, and you struggle to find a single one in which the more charismatic candidate didn’t win. Voters care about issues and ideology but, more than anything else, they wind up choosing the person they find most compelling. Charisma comes in many forms, both hopeful (e.g., Barack Obama) and poisonous (e.g., Donald Trump), but if you can’t make people get up out of their seats and cheer, you haven’t got much of a chance.

Ask reporters who have been out on the campaign trail this year, and they’ll tell you that Warren’s rise has not surprised them because, from the beginning, she has been the candidate who seemed to excite voters the most. That, too, is no guarantee of victory — in many polls, she still trails former vice president Joe Biden — but you definitely need it if you don’t have a lot of other cards to play.

Timing is (almost) everything. With 20/20 hindsight, we can look back at almost every presidential primary campaign and say, “Well, of course that candidate became the nominee.” Parties evolve and react to events, and wind up choosing the person who most embodies who voters are and what they want at a particular moment. In 2016, Republicans wanted an angry xenophobe who promised to restore the America of their youth. In 2008, Democrats wanted a young, cool, multiracial cosmopolitan urbanite who said that their America was the truest one. And what do Democrats want now?

They’re still working that out, but the central question is whether they want someone offering a reassuring incrementalism they hope will be less threatening to Republicans but might not inspire adoration among their own base, or someone offering a more ambitious agenda of change that will excite their voters but might not appeal as much to those who don’t already like them.

The candidates who have fallen by the wayside — and those who are still in the race but can’t seem to win over large numbers of voters — are the ones who have been unable to make the most compelling case for one of those two approaches. The argument still hasn’t been settled.

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