What was Mitch McConnell thinking? The Senate majority leader, who's widely respected as one of the shrewdest operators in Washington, seemed to make a pretty big mistake when he announced soon after Antonin Scalia's death that Senate Republicans won't even consider a nominee President Obama makes to fill Scalia's seat on the Supreme Court — no confirmation hearings, no votes, nothing. In short order, every Republican within reach of a microphone signed on to McConnell's obstruction plan. In doing so, they made their party look petty and intransigent, seeming to say that if they aren't guaranteed to win, they're going to take their ball and go home.

There was an easier way: Let Obama nominate someone, and then reject him or her. It wouldn't have been hard, since Republicans enjoy a 54-46 advantage in the Senate and are likely to face few if any defections from opposition to whoever Obama nominates. They could have pretended to give the nominee all due consideration, then just voted him or her down. Instead, they showed their hand right at the beginning.

People in both parties seem to agree that Scalia's death makes the Supreme Court an even bigger issue in the presidential campaign than it was before. Just how big? It's impossible to say just how big quite yet, but most of the benefits will go to the Democrats.

The presidential candidates in both parties will argue, quite rightly, that their partisans had better get out and vote or watch the Court slip away from them, with dire results. But Democrats have a better chance of persuading voters in the middle that a Supreme Court tilted in their direction would be a good thing. While Republicans have gotten lots of victories from the Court in recent years on issues like guns, voting rights, corporate privileges, and affirmative action, when you ask them what issue will be most important at the Court in coming years, they're likely to cite abortion. Which is exactly what Democrats will say as well. And with the exception of Donald Trump (who has been vague on the issue) all the Republican candidates have proclaimed their commitment to overturning Roe v. Wade, a deeply unpopular position shared by only around 30 percent of the public. They'll also tell their base that they want to roll back the decision legalizing same-sex marriage — which would also be tremendously unpopular.

Beyond that, the controversy over Scalia's replacement will play out over the coming months, and not to Republicans' advantage. Their argument — that Barack Obama has no right to appoint Scalia's successor since he won't be in office a year from now — isn't just inane on its face, it's also likely to be seen by African-American voters as one more example of Republicans disrespecting the first black president. Keeping those voters motivated and voting almost unanimously for Democratic candidates is a key task for Democrats as they try to hold the Obama coalition together, and it could matter in swing states like Michigan (which is 14 percent African-American), Florida (17 percent), and Virginia (20 percent).

Then there's the matter of the particular individual Obama nominates. If we assume that Republicans hold to their promise not to consider the nominee, he or she will be something of a sacrificial lamb (unless a Democrat wins in November and re-nominates the same candidate). So if Obama wanted, he could pick a nominee whose poor treatment by Republicans would reinforce the Democratic narrative that the GOP is a party of old white guys who are hostile to rising minority groups gaining any power and influence. In their presidential primary campaign, Republicans have already worked hard to alienate as many different minority groups as possible; now add to that the spectacle of a potentially trail-blazing Supreme Court nominee — a Latino or an Asian-American, for instance — being held in limbo by the Republican Congress, all while the party's nominee tries in vain to convince those same minority voters that they can find a home in the GOP.

And it isn't just the presidential race that could be affected. Democrats need a net gain of five seats to take back control of the Senate, and most handicappers judge that task to be difficult but possible. Five of the most vulnerable Republicans up for reelection are from states that Obama won in both 2008 and 2012. Facing electorates that lean Democratic, these senators want to avoid any controversies that make them look like captives of a radical GOP. As of this writing, four of the five — Kelly Ayotte (New Hampshire), Rob Portman (Ohio), Pat Toomey (Pennsylvania), and Ron Johnson (Wisconsin) have endorsed McConnell's no-consideration plan, while Mark Kirk (Illinois) has been avoiding reporters. Their Democratic opponents will no doubt be eager to make their obstructionism an issue.

For all these reasons, the Supreme Court looks like it could be one of the most critical issues in the fall campaign. And that's as it should be, both because the Court affects all Americans' lives in profound ways, and because, as I've been arguing for some time, it's poised for a dramatic shift. If a Democrat appoints Scalia's successor, it will be the first time since Clarence Thomas replaced Thurgood Marshall in 1991 that a vacancy shifted the balance of the Court (although you might count Samuel Alito replacing Sandra Day O'Connor). Not only that, two justices, Ruth Bader Ginsburg and Anthony Kennedy, will be in their 80s when the next president takes office, and another, Stephen Breyer, will be 78. The Supreme Court we see at the end of the next president's term could be dramatically different than the one we have now, with consequences that could reverberate for a generation or more; the only question is which direction it will move.

That's something every voter should care about.