The Supreme Court’s new term that begins on Monday could prove momentous, with cases involving gun control, abortion, L.G.B.T. rights and immigration. The term is likely to signal how far the court’s conservative majority will go to block the agenda of the next Democratic president and Congress.

With the court on the precipice of a dangerous lurch rightward, polling data indicate that Democrats have a positive view of Chief Justice John Roberts, who has expressed regard for precedent and concern for the court’s legitimacy, encouraging a view that he will step in to prevent partisan excess. Yet history suggests that Democrats have much to fear. The chief justice is neither a swing vote among his four liberal and four conservative justices, nor a moderate. Expect him to land time and again with the conservatives.

During his 2005 confirmation hearing, Chief Justice Roberts affirmed his “respect for precedent, even-handedness, predictability, stability.” But once on the bench, he compiled a voting record that is among the most partisa n of any justice in the modern era when it comes to cases overturning precedent, according to our analysis. He has presided in 21 such cases and voted to overturn precedent in 17, or 81 percent, making him the second behind only Justice Clarence Thomas as the most frequent member of a precedent-overturning majority over the last 14 years. In 15 such cases with partisan implications , the chief justice voted for conservative outcomes 14 times, or 93 percent of the time. In determining whether a decision was liberal or conservative, our organization, Take Back the Court, relied on the methodology used by the Supreme Court Database, maintained by the law school at Washington University in St. Louis. To determine whether an outcome was partisan, we assumed that if at least four liberal justices voted for a decision in a split ruling, then the outcome was consistent with Democratic Party values and goals. And if at least four conservative justices voted similarly, then the outcome was considered consistent with Republican Party values and goals.

Our analysis shows that the chief justice’s voting record is as conservative as those of his most extreme current and former colleagues — Justices Thomas, Samuel Alito, Brett Kavanaugh and Antonin Scalia. In all 42 split-decision cases that the chief justice has presided over involving racial minorities, immigrants, workers and abortion, he voted for conservative outcomes 100 percent of the time.