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It took conservatives 50 years to get a reliable majority on the Supreme Court. Here are 3 reasons why.

Supreme Court Justice Anthony M. Kennedy, 81, said he is retiring. The Post's Robert Barnes explains why that will ignite a "real fight" for his replacement. (Video: Monica Akhtar/The Washington Post, Photo: Jabin Botsford/The Washington Post)

Justice Anthony Kennedy’s retirement, announced Wednesday, will give President Trump and a Republican Senate the chance to cement a reliable conservative majority on the Supreme Court for the first time since the New Deal began. For 50 years, they seemed poised to accomplish this feat and often came close. Why did it take so long?

The answer, as I’ll explore below, involves the politics of appointments, justices’ “ideological conversions,” and Senate rules. Nor is it clear that the court will shift as far right as some are now predicting, given the risk of public backlash.

GOP presidents haven’t always appointed conservative jurists

Richard Nixon’s election in 1968 launched an era of Republican domination of Supreme Court appointments. Nixon and subsequent GOP presidents made appointments a campaign issue, pledging to nominate conservative, “strict constructionist” jurists. The figure below shows presidential appointees from Nixon to Trump, including whether they were appointed during unified or divided government (i.e., whether one party held both the presidency and the Senate), and whether they ended up reliably conservative (shaded red), reliably liberal (blue), or a “swing justice” (no shading).

The latter coding is rooted in research on justices’ ideology and voting, as well as coverage of Justice Neil Gorsuch’s short tenure. Liberal and conservative positions are defined in accord with the Supreme Court Database.

Republican presidents have had ample opportunity to move the court to the right. They made 10 appointments between 1969 and 1992 — that’s every new justice added during those years — and then three more during the George W. Bush and Trump years. That’s 13 GOP-appointed justices since 1969. Democratic presidents Bill Clinton and Barack Obama appointed only four.

What’s more, nearly half of the 13 GOP nominees replaced justices who’d been appointed by Democratic presidents.

And yet the court’s ideological tenor did not change as much as expected. Since the 1970s, studies show a generally moderate to center-right court. In fact, in contemporary times, the court has issued more liberal rulings in its most high-profile cases than conservative rulings, such as decisions to uphold the constitutionality of the Affordable Care Act, the legalization of same-sex marriage and restrictions on the death penalty.

Here are three reasons the court didn’t move more quickly to the right:

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Divided party control of government in the past limited GOP gains

While past presidents often subscribed to non-ideological selection norms (such as region, gender, religion and experience), divided government hurt GOP presidents’ efforts to stack the court with reliable conservatives. Republican presidents were forced to nominate judges who could get confirmation votes from both sides of the aisle.

For example, a Democratic Senate rejected two of Nixon’s nominees (1969 and 1970), and more prominently, it rejected Ronald Reagan’s nomination of conservative Robert Bork in 1987. After Douglas Ginsburg withdrew, Reagan filled that seat with Justice Kennedy. Eight of the 10 justices appointed between Nixon and George H.W. Bush had to be confirmed by a Democratic Senate. The last seven justices were appointed when the president’s party controlled the Senate. Not coincidentally, all seven are considered reliable ideologues.

Party control of the Senate is now even more important to the president’s ability to shape the court than it was before the 1990s. In 2016, Senate Republicans refused even to consider Obama’s nomination of Judge Merrick Garland to the court, arguing that the new president should make the nomination. Had Obama been able to replace Justice Antonin Scalia — whose death left the vacancy — with Garland, the president could have created the first reliable liberal majority since the Warren Court.

Ideological conversions among Republican justices

Justices selected by Republican presidents over the past half-century haven’t always turned out to be so conservative.  Judicial scholars call this “ideological drift”: when justices’ policy positions over the course of their careers move away from initial expectations.

Three Republican appointees — Justices Harry Blackmun, John Paul Stevens and David Souter — underwent wholesale ideological reversals, drifting from center-right to liberal positions. Justices Sandra Day O’Connor and Kennedy became quintessential “swing justices,” casting decisive votes in significant liberal and conservative rulings.

Ideological drift clearly delayed a conservative court majority, despite the fact that Republican presidents have dominated appointments over the past half-century.

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New Senate rules facilitate a polarized court

Before 2017, Senate rules required a supermajority to cut off debate on Supreme Court nominations (three-fifths of the votes, rather than half plus one). That meant that even when a president’s party controlled the Senate, presidents were likely to choose nominees who could attract at least a handful of minority-party senators. If those rules were still in place now, a Democratic filibuster could delay consideration until after the midterms, when the Democrats may control the Senate.

But last year, facing a Democratic filibuster of Gorsuch, Senate Republicans eliminated the filibuster for Supreme Court nominees; only a majority is now needed. That means a reliable conservative will almost surely be confirmed, which is particularly important for a “critical nomination” like this one, which could change the court’s ideological balance of power.

Will the risk of public backlash move Roberts to the center?

If President Trump appoints — and the Senate confirms — the reliable conservative who most observers expect, that person would create the most ideologically polarized court in history. On each side, for the first time, we will see ideologically homogeneous (and opposed) liberal and conservative coalitions, with no justice in the center.

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But that doesn’t guarantee that the court will leave a reliable conservative imprint on the law on all issues. Because the court has been generating a fairly even balance of liberal and conservative rulings since the 1970s, it experiences more transitory criticism of its legitimacy (or “rightful authority”) that is appeased when the court delivers a pleasing ruling.

That could change if a polarized, conservative court delivers an onslaught of conservative policy-making. The country could see increased attacks on the court’s authority, including attempts at subverting partisan rulings, demands that the court’s ability to decide certain issues be stripped, and calls for term limits or even retention elections.

Chief Justice John G. Roberts Jr. has emphasized the importance of building consensus and thus legitimacy. Some observers say that such considerations swayed Roberts to twice uphold the ACA. O’Connor and Kennedy, earlier in their careers, may have inched toward the center precisely to help the court stay more moderate so that it could withstand sustained attacks on its legitimacy. Could Roberts’s concern about a polarized bench edge him to the center to protect the court’s public standing?

Apparently, we will find out.

Brandon L. Bartels is an associate professor in the department of political science at George Washington University.