Barack Obama has seen a few confrontations during his time in the White House but this one is historic. 

Every part of the American body politic is involved: the presidency, the Senate, the Supreme Court and the people. 

As his presidency winds down, the executive branch is careening toward a collision with the legislative branch over an appointment to the judicial branch — and in the middle of an election campaign.

How the country moves forward in the next few decades will depend on the composition of the U.S. Supreme Court, and the composition of the court depends on the president who appoints the justices. 

Only hours after Antonin Scalia's death on Saturday, Republican Senate Majority Leader Mitch McConnell was throwing down a gauntlet.

Justice Scalia's seat on the court should stay vacant until after the election, he said, so that the people can choose the president who nominates the judge's successor. It was strongly suggested McConnell will refuse to hold hearings on a nominee.

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President Barack Obama has said he seeks to fill the U.S. Supreme Court seat left vacant by the death of Justice Antonin Scalia, setting up a certain election-year showdown with the Republicans. (Pablo Martinez Monsivais/Associated Press)

The president picked up the gauntlet that same evening and said he would be sending his nomination for Scalia's replacement to the Senate for confirmation soon. 

It looks as though the defining issue of the election campaign has arrived.

Republican presidential candidate Senator Ted Cruz said Saturday, "We're not going to give up the U.S. Supreme Court for a generation by allowing Barack Obama to make one more liberal appointee."

That's not just Ted being Ted; Democrats agree this is a struggle for the ages.

It was inevitable the presidential campaign would explode like this because so many of the judges on the court are so old: Scalia was 79, Stephen Breyer is 77, Anthony Kennedy is 79, and Ruth Bader Ginsburg is 82.

The next president might have to replace all four, something that hasn't happened since Richard Nixon was in the White House. 

Key cases coming up

What it means for the future can be gleaned from the past. Look at how consequential and divisive some of the court's recent decisions have been.

With some of its narrowest rulings — those decided by a 5-4 vote — the U.S. Supreme Court has confirmed the legality of Obamacare, legalized same-sex marriage and gutted campaign finance laws.

In the immediate future it will consider abortion, immigration, birth control and affirmative action. 

These are not arcane issues for ordinary people.

So the Republicans know that by blocking Obama's court nominee in the Senate, they risk creating an issue that might hand the presidency to the Democrats in November.

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A group with 'People for the American Way' gather with signs in front of the U.S. Supreme Court in Washington, calling for Congress to give fair consideration to any nomination put forth by Barack Obama to fill the seat of Antonin Scalia. (Carolyn Kaster/The Associated Press)

Consider how some of the court's decisions have already played out on the stump.

Democratic presidential contender Senator Bernie Sanders has attracted an enormous following largely by attacking the Supreme Court's recent decisions on deregulating election campaign spending laws.

In one case, Citizens United vs. FEC, the Republican appointees on the court found that corporations have as much right to free speech as actual living, breathing people.

In another case, McCutcheon vs. FEC, the same judges declared that money has no corrupting influence in politics.

It's easy to imagine how the lanky, white-haired Sanders will play the latest news.

Leaning forward over his podium, elbows up, both arms reaching out to stab the air with long fingers, he'll warn that, if they get the chance, Republicans will only appoint more justices like the ones they appointed before — the ones who have already allowed corporations and billionaires to stomp around in heavy boots, trampling the sacred garden of democracy.

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With many Supreme Court cases recently decided by a 5-4 margin, with Antonin Scalia leading the conservative majority, the next appointment could mean a shift in the balance of power. (Alex Wong/Getty Images)

The prose will only turn deeper purple when Obama reveals his nominee. Suddenly there will be a name, a face and a judicial record to gin up his case as well. 

Whoever the nominee is will undoubtedly say the court was wrong on the campaign finance rulings.

Republicans aren't likely to want to run against that. They can see the reaction Sanders gets from Democrats when he decries the impact of Republican judges on the country's politics.

It's the same reaction Donald Trump gets from the Republican base when he rails against billionaires and the pernicious influence of their money on democracy.

Unwritten 'Thurmond Rule'

So of course the Republicans are trying to reframe the issue of appointments as an issue of principle instead.

Obama is too close to the end of his presidency to be allowed a consequential decision such as the lifetime appointment of a top court judge, they say.

Democrats made the same argument near the end of George W. Bush's presidency and the case seemed weak then too. The U.S. Constitution does not define a period after which the president loses his authority to appoint.

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Republican presidential candidate Senator Ted Cruz has said he will filibuster any Supreme Court nominee made by outgoing U.S. President Barack Obama. (Joshua Roberts/Reuters)

There is murmuring about a so-called "Thurmond Rule" that supposedly prohibits Senate confirmation of appointments six months before the end of a president's term. Or is it after June in the president's final year?

No one knows for sure because the rule isn't written down anywhere, certainly not in the constitution.

It doesn't even seem as though it would apply, given that a confirmation could be all done and dusted before either of the Thurmond clocks ran out.

But expect to hear more about all of it this political season.

And don't be shocked if the U.S. Supreme Court still has a vacancy a year from now. Or if McConnell, having won his battle with Obama, nevertheless has to confirm the Obama nominee in 2017, under the next president.