Keith Boykin, a former White House aide to President Clinton and the author of four books, teaches a course on African American Politics at Columbia University.

Every four years, presidential candidates warn their supporters that the Supreme Court is on the line. The argument rarely works since voters know there's no guarantee a particular president will enjoy the opportunity to appoint a justice, much less one who might alter the balance of the court.

This year could be very different. If Senate Republican leader Mitch McConnell and Senator Ted Cruz carry out their threats to deny President Obama's Supreme Court nominee a chance for a vote, their obstructionism could be just the motivation Democrats need to drive reluctant voters to the polls in November. That would endanger the G.O.P. nominee and the seven G.O.P. incumbents who are up for reelection this year in states Obama carried in 2012.

This obstructionism could be just the motivation Democrats need to drive reluctant voters to the polls in November.

Just visualize the optics of McConnell and Cruz blocking the nomination of, say, a qualified woman of color for the Supreme Court. Republicans this primary season shattered voter turnout records in the Iowa caucuses and the New Hampshire primary while Democrats didn't quite match the enthusiasm of the 2008 election. Add in several more months of a bruising presidential nominating process between Bernie Sanders and Hillary Clinton, and Democrats may need a common adversary to unite them for the general election. Enter Ted Cruz, Mitch McConnell and the G.O.P. presidential nominee.

Much of this year's presidential campaign has been animated by the sentiment that both political parties are broken with few significant differences between them. A battle to appoint a progressive Obama nominee to the high court would repudiate that notion of false equivalence promoted by some on the far left by highlighting sharp distinctions between the parties. On issues of contraception, abortion, immigration, L.G.B.T. rights, voting rights and affirmative action, it really does matter which party controls Congress and the White House.

Mitch McConnell's naked power grab, coming on the heels of a government shutdown, more than 60 failed attempts to repeal Obamacare, and numerous other manufactured crises, also helps refute the unsubstantiated media narrative that "both parties are to blame" for the gridlock in Washington. There's no precedent in modern history for denying the duly elected president of the United States a hearing or a vote on a Supreme Court nomination. This level of obstruction exposes the G.O.P.'s pattern of intransigence and could push independents to the Democratic fold in the fall.

It's a move Republicans will live to regret.



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