Finally, Alito authored two majority opinions that questioned a nearly 40-year-old Supreme Court precedent that allows public-sector unions to collect a fair share of the cost of collective bargaining from all government employees who benefit from that bargaining, even if they’re not union members. Alito’s push to overturn this landmark decision represents a major threat to public-employee unions nationwide. And no doubt thanks to Alito’s encouragement, the Court is currently considering whether to overrule that 40-year-old precedent in Friedrichs v. California Teachers Association.

In the end, both Roberts and Alito are solidly conservative justices—willing to use a range of interpretative tools to push the law to the right in a number of critical areas. But there is one significant difference between the two justices, and that difference helps explain why their records have been materially different in their first decade on the Court. As we concluded after taking a year-long look at Roberts’s first decade as chief justice, Roberts is deeply concerned about the institutional legitimacy of the Court—since joining the Court, he’s repeatedly expressed the concern that the Court not be seen as an extension of the political process—and there are areas in which that concern trumps his ideology. In contrast, Alito’s record is almost always consistent with his conservative ideology.

Perhaps the most prominent examples of this Roberts-Alito split are the two health-care reform cases in which Roberts, but not Alito, voted to uphold, at least in substantial part, the Affordable Care Act. In the first case, Roberts cast the deciding vote to uphold the ACA’s individual mandate against constitutional challenge and, in the second, he joined Justice Kennedy and the Court’s more progressive members to hold that the tax subsidies that put the “affordable” into the Affordable Care Act should be available nationwide. Alito dissented in both cases.

Roberts and Alito were also on opposite sides in United States v. Arizona, another case in which the institutional stakes for the Court were high. In that case, Roberts joined Kennedy’s opinion striking down key parts of Arizona’s anti-immigrant law, protecting the executive branch’s broad discretion when it comes to enforcing the nation’s immigration laws. (This decision is of particular salience now given the Court’s decision to hear the challenge to President Obama’s executive action on immigration this Term.)

By our count, there have been at least 10 significant cases in which Roberts parted ways with at least some of his conservative colleagues (including Alito), which is why, as one of us wrote last year, “in many important cases, progressives shouldn’t count him out.” The same can’t be said for Alito. This is not to say that Alito never votes for a result that seems to be at odds with his ideological views—he has on occasion (for example, he voted with the rest of the Court to hold that warrantless cell-phone searches following an arrest are unconstitutional, though he wrote separately and by himself to suggest that state legislatures could choose to allow such searches)—but such instances are exceedingly rare.

Even Justices Scalia and Thomas have areas (most notably, some areas of criminal procedure) in which their votes are clearly in play. But it’s difficult to say what those areas or cases might be when it comes to Alito. And that’s surely why, 10 years into his tenure, it appears there’s no one to the right of Alito on the current Court.

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Tom Donnelly is senior fellow for constitutional studies at the National Constitution Center.