This is The Big One.

​Replacing retiring Supreme Court Justice Anthony Kennedy will probably be the most consequential decision President Donald Trump makes in his first term.

​During Kennedy’s 31 years on the bench, he was the court’s swing vote. Especially in the most controversial cases, he was usually the reason a decision went 5-4 … or 4-5. With Kennedy’s departure, the Court is currently split between four “textualist” conservatives, who base their judgments on what the Constitution says, and four progressives, who base their judgments on what they want the Constitution to say.

​And so President Trump and the Republican majority in the U.S. Senate now have a once-in-a-generation opportunity to cement a clear conservative majority on the Supreme Court.

​The left knows it. The right knows it. And both sides also know the stakes.

For decades, Congress’s abdication of its constitutional responsibilities has turned the Supreme Court into a super-legislature. This long era of judicial supremacy has been bad for America, but it has been very good for the Democrat Party.

​From abortion to gay marriage to the extra-constitutional power of the federal bureaucracy, the Left’s greatest victories in recent generations have come not via the ballot box, but the courts. Under liberal legal ideology, every federal judge is a Congress unto him — or herself — free to rewrite federal law, and even the Constitution, as they see fit. The “living Constitution,” they call it.

A conservative, textualist majority on the nation’s highest court would put an end to that. It would not — and should not — start imposing conservative policies on the country. Rather, it would get the courts out of the policy business altogether, and leave that job of lawmaking to lawmakers and voters where it belongs.

To most Americans, this level political playing field seems fair. To the left — the ones who have been tilting the playing field all this time — fairness is an existential threat. That’s why the coming Supreme Court confirmation process is going to be toughest and most important political fight most of us have ever seen.

​Luckily, we have a fighter in the Oval Office ready to lead it. And it seems to me that President Trump needs another fighter in the nominee him — or herself.

This is crucial. We all know failure is not an option this time. But there are two kinds of failures to avoid here. The first, obviously, is choosing a nominee who divides Republican Senators and grassroots conservatives, and cannot win confirmation. The second and greater threat is choosing a nominee who seems conservative now … but has the potential to “grow in office,” as they say in The Swamp.

​To win on the Supreme Court — where Republican presidents before him all failed — President Trump needs to pick a justice who can be as tough as he is, and has proven it. One obvious candidate he should strongly consider is Senator Mike Lee of Utah.

I served with Mike in the Senate, and he’s a good friend. He has exactly the brilliance, the background, and the temperament that President Trump should be looking for. But he also has that rarest of qualities: courage — the willingness to do the right thing, no matter what. Mike has the battle scars to show he doesn’t back down from a fight — including countless fights with his own party’s Washington establishment. And just as importantly, Mike has waged every fight with the kindness and generosity and thoughtfulness that define his character. He has the mind to dominate his confirmation hearing, and the character to even win a few Democrat votes.

President Trump has assembled a terrific list of smart, capable candidates, and all have the potential to make good on his promise to remake the Court. They all have the right convictions. But in the Swamp — especially in a lifetime appointment — it’s often the courage of those convictions that matters most. Donald Trump of all people knows that. When he makes his decision, we know whoever he nominates is going to have the toughest fighter in American politics in his corner.

But for the political fight of the century, President Trump should put a fighter in the ring, too.

Jim DeMint is chairman of the Conservative Partnership Institute and a former U.S. senator representing South Carolina.

The views and opinions expressed in this commentary are those of the author and do not reflect the official position of The Daily Caller.