The announcement by Justice Anthony M. Kennedy that he will retire from the Supreme Court at the end of July had the effect on the American political conversation of hitting a hornet’s nest with another hornet’s nest. Instantly, speculation flew in a million directions with the immediate expectation that the next few months of politicking over President Trump’s choice to replace Kennedy would be ferociously fought.

It’s worth noting, though, that Kennedy’s last term on the court saw him joining with its conservative justices far more than he did with the liberals. Put another way, whoever replaces Kennedy — almost certainly a staunch conservative — would have voted the same way Kennedy did this term on the vast majority of cases.

SCOTUSBlog tracks the frequency with which justices vote with one another. The darker the block on the chart below, the higher the percentage of the time the two justices agreed fully or partially on decisions.

You can see the two factions: Justices Sonia Sotomayor and Ruth Bader Ginsburg voted together 93 percent of the time. Justices Samuel Alito and Clarence Thomas voted together 90 percent of the time. Sotomayor and Alito? Only 8 percent of the time, often on unanimous decisions.

If we break out Kennedy’s line by faction, his alliance with the conservatives is clear. He voted with the four more conservative justices about three-quarters of the time; with the liberals, less than half.

This was a break from Kennedy’s past voting patterns, as The Post’s Joe Fox, Ann Gerhart and Kevin Schaul note. From 2014 to 2017, he was more likely to vote with the liberals than the conservatives. In the most recent term, he voted with the conservatives more frequently than he had in the previous decade.

It’s the exceptions, though, that are important — critically so.

There were a number of significant cases this term, but none on abortion, for example. Kennedy joined with former justices Sandra Day O’Connor and David Souter in 1992 to write the opinion in Planned Parenthood v. Casey upholding Roe v. Wade. The New Yorker’s Jeffrey Toobin offered a prediction on Twitter: “Abortion will be illegal in twenty states in 18 months.”

It’s a virtual certainty that whoever replaces Kennedy will support overturning Roe v. Wade. While Trump himself was once publicly supportive of the right of women to seek abortions, he’s made delivering for his staunchly conservative and evangelical base central to his presidency. He trumpeted both his opposition to abortion and his intent to appoint conservative justices as selling points on the campaign trail. It paid off; as we noted Tuesday, a quarter of his voters cited the ability to appoint Supreme Court justices as the most important factor in their 2016 votes.

In the final presidential debate with Hillary Clinton, Trump was explicit. Would he want the court to overturn Roe v. Wade?

“Well,” Trump replied, “if we put another two or perhaps three justices on, that’s really what’s going to be — that will happen. And that’ll happen automatically, in my opinion, because I am putting pro-life justices on the court.”

Kennedy voted with Neil M. Gorsuch, Trump’s first nominee to the Supreme Court, three-quarters of the time. That’s more than Gorsuch voted with the other conservatives. He voted with the conservatives on most critical decisions this term, such as upholding Trump’s travel ban, rejecting public union agency fees and  supporting the baker who declined to bake a cake for a same-sex wedding. Whoever replaces him would probably have joined him in those votes. He’s been a swing vote in the past but played that role to a lesser extent in his final term.

Where his absence will be felt is not in most cases. It will be felt the most in the most important cases.

Update: There’s another more subtle factor, too, noted by Court-watcher Jay Pinho.

“Justice Kennedy may have been particularly unhelpful to progressives in this past term, but the greater threat is in what cases make it to the Supreme Court in the first place,” he wrote. “Assuming he is replaced by a consistent Gorsuch-style conservative, right-wing plaintiffs will be emboldened to take increasingly radical cases to the Court in hopes of overturning longstanding precedent and setting new ones entirely.”

In order to take a case, four of the nine justices must vote to do so. If, in the past, Justices Gorsuch, Alito and Thomas wanted to hear a case on abortion rights, they would have had to find a fourth vote to join them — a vote that wouldn’t have come from Kennedy.

It likely would come from his replacement.