Casey is the total package — assuming the Democrats’ idea of a total package is mild, earnest and quite possibly the most boring person in politics.

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This is why they should instead go with Sen. Cory Booker (N.J.) — unless Democrats are concerned about their standard-bearer being a showboat who declares, with a straight face, “I am Spartacus.”

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In this case, I say: Go with everyman Joe Biden — as long as they won’t mind putting a 77-year-old former vice president up against a 74-year-old incumbent.

Otherwise, the best choice would be youthful Rep. Eric Swalwell (Calif.), though the 38-year-old’s main qualification is cable-news overexposure.

Hence, the need for a candidate of substance: Sen. Amy Klobuchar (Minn.) — as long as her perceived lack of toughness isn’t a problem.

If it is, combative Sen. Kamala Harris (Calif.) is the best option. But she’s not the person you’d want to have a beer with.

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No, that person is Gov. John Hickenlooper of Colorado, who once ran a brewery. Except his name makes everybody giggle.

Better, then, to go with another white guy from Colorado, Sen. Michael Bennet — assuming the electorate is as excited as he is by filibuster reform.

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If not, the fiery populist Sen. Elizabeth Warren (Mass.) is top choice — unless Democrats think her DNA-test adventure means President Trump has her number with “Pocahontas.”

If genuineness is the essential quality, Rep. Beto O’Rourke of Texas is the man, though he is a failed Senate candidate, which also probably knocks out Richard Ojeda (W.Va.), a failed House candidate.

Therefore, may I suggest Sen. Sherrod Brown or Rep. Tim Ryan, who win among Ohio steelworkers? Except there aren’t that many such workers left.

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The Latino population, by contrast, is large and growing, making former housing and urban development secretary Julián Castro ideal. If only Latinos turned out to vote.

This is why I would suggest an outsider, such as billionaires Howard Schultz (except he’s not ready to mix it up with Trump), or Mark Cuban (except he’s another reality TV guy), or Tom Steyer (except he, like Washington Gov. Jay Inslee, is a one-trick pony on climate change).

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Ergo, the optimal choice is Michael R. Bloomberg — only taller. Or Bill de Blasio — only shorter.

Sens. Bernie Sanders (I-Vt.) or Tim Kaine (D-Va.) or former Maryland governor Martin O’Malley would be perfect — if they weren’t 2016 retreads.

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Hillary Clinton herself would be ideal — if she weren’t a 2016 and a 2008 retread.

John Kerry would be the logical pick — if he weren’t a 2004 retread.

And Gov. Jerry Brown (Calif.) would be the man — if he weren’t a 1976, 1980 and 1992 retread.

I therefore nominate former attorney general Eric H. Holder Jr. — though he’s an Obama administration retread. My fallbacks, then, are former governors Deval Patrick (Mass.) and Terry McAuliffe (Va.) — though they’re both Clinton retreads.

May I propose, then, that Democrats go with a storied name, such as Rep. Joe Kennedy (Mass.) or Gov. Andrew Cuomo (N.Y.)? Although the Clintons may have killed dynastic politics.

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If that’s the case, I advocate one of these self-made legislators: Sens. Mark Warner (Va.), Jeff Merkley (Ore.) or Kirsten Gillibrand (N.Y.); or Reps. Adam “Little Schitt” Schiff (Calif.), Seth Moulton (Mass.), Tulsi Gabbard (Hawaii) or John Delaney (Md.). But, come to think of it, their congressional voting records would be easy to distort, and they are vulnerable to the charge that they don’t “run” anything.

Hence, my first choice: Montana Gov. Steve Bullock. Except he’s unlikely to turn out the 99 percent of Democrats who don’t live on farms.

This leads us to former New Orleans mayor Mitch Landrieu, or Los Angeles Mayor Eric Garcetti — except Democrats need to appeal to people who live beyond city lines.

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The ultimate pick, therefore, is Mayor Pete Buttigieg of South Bend, Ind., a gay Democrat in a conservative area. Or, he would be the ultimate pick if he weren’t utterly unknown.

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No one is better known than Oprah Winfrey, who would be terrific — if only she’d run. Otherwise, Democrats can have Oprah’s friend, spiritual guru Marianne Williamson, who just announced that she’s running.

Democrats, however, are not feeling their inner peace and harmony at the moment, so probably they should skip Williamson and go with a street brawler, Michael Avenatti — assuming he isn’t himself among the prison population on Election Day.

Which is why the Democrats need somebody sober, predictable and even boring — such as Bob Casey.

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Twitter: @Milbank