You know what the Democratic debates need more of? Republican talking points.

That probably wouldn't be a popular position with the 10 candidates onstage Wednesday night during the Democratic presidential debate in Detroit. Pressed to compare and contrast themselves and their policies against each other, candidates like Sens. Kamala Harris (D-Calif.) and Cory Booker (D-N.J.) repeatedly complained that the questions were somehow unfair.

Challenged by Colorado Sen. Michael Bennet on the details of her health-care plan, Harris complained that the critique was an echo of "Republican talking points." Booker went further, grumbling that "this pitting against progressives against moderates … that to me is dividing our party and demoralizing us in face of the real enemy here." Booker's campaign even bought a Twitter ad during the debate to amplify his complaint the debate was somehow playing into the hands of President Trump.

All of this, of course, echoed the first night's debate, when Sens. Bernie Sanders (I-Vt.) and Elizabeth Warren (D-Mass.) grumbled about being asked to respond to "Republican talking points," with plenty of activists joining in online. "No Democrat wants to watch our candidates savaging one another," one wrote.

Newsflash to candidates: Pitting you against each other is precisely the reason debates and primary elections exist. So stop complaining, and get to fighting.

Democratic voters are going to have to winnow a field of more than 20 candidates down to one nominee. General election voters will have to select a president from two major-party candidates, assuming they skip right past the innumerable minor-party nominees. They have to decide which candidate is best prepared for office and which campaign offers the smartest ideas.

It's not a unifying process — it can't be, if voters are going to do their job.

And yes, that means that it is fine and good for Democrats to face critiques that Donald Trump might use against them during the general election. Why? Because Donald Trump might use those critiques against them in the general election. If Democratic candidates can't face the heat now, the president will make mincemeat of them by the time Fall 2020 rolls around. The party's voters deserve to know now who can withstand "Republican talking points," because they're coming.

So it's time for the candidates to toughen up.

We received a clear example of this dynamic in the 2016 election. During the primaries, Sanders famously passed on an opportunity to criticize the eventual nominee, former Secretary of State Hillary Clinton, for her use of a private email server to conduct government business. "The American people are sick and tired of hearing about your damn emails," Sanders said, admitting that foregoing the issue probably wasn't "great politics."

His decision certainly didn't help her. Trump didn't hesitate to bring the issue up during the general election, and with the FBI investigating the matter until deep into the campaign, the issue remained a live wire for Clinton to the very end.

Which means that former Vice President Joe Biden has to have answers for complaints he looked the other way while President Obama conducted record deportations. Harris has to do a better job explaining her prosecutorial record. And Booker must be able to defend his time as mayor of Newark.

Indeed, for all his complaints about the divisiveness of the debate, Booker himself may have shone brightest when he was most aggressive — like when he challenged Biden to discuss Obama's deportation record.

"You can't have it both ways," Booker said. "You invoke President Obama more than anyone in this campaign; you can't do it when it's convenient and then dodge it when it's not."

It was a good point. It was the kind of point Trump will make next year if he faces Biden in the general election. Biden had best get his response together now.

Democrats can sometimes be too timid — worried about "electability" instead of who they most want to support for the White House, for example. Sometimes, they worry so much about the critiques, about the downsides of battling each other, that they fail to consider that putting forth a battle-tested nominee might be the best way to win the presidency.

The best thing they can do is fight now, then unify when a nominee has been chosen. Any Democrat who wins the presidency will have a fight on his or her hands. They might as well take a few shots from their friends first.