IN early 2009, as Barack Obama was about to take office, Mitch McConnell, the leader of the Republican minority in the Senate, assembled his caucus at a retreat in West Virginia. There, he laid out his strategy for taking on the new president, who was sweeping into office on a tide of popularity, historical resonance and great expectations barely diminished by the economic free fall then underway.

The key, Mr. McConnell told his fellow Republicans, was to stymie and undermine Mr. Obama, but to do so in subtle ways. As one of the senators present, Robert F. Bennett of Utah, later recalled to me: “Mitch said, ‘We have a new president with an approval rating in the 70 percent area. We do not take him on frontally. We find issues where we can win, and we begin to take him down, one issue at a time. We create an inventory of losses, so it’s Obama lost on this, Obama lost on that. And we wait for the time where the image has been damaged to the point where we can take him on.’ ”

Seven years later, with the Republicans now in the Senate majority, the opposition led by Mr. McConnell is as frontal as can be. After word of Justice Antonin Scalia’s death emerged last weekend, it took the majority leader less than an hour to announce that the Senate would not entertain a replacement before November. “This vacancy should not be filled until we have a new president,” he said.

Mr. McConnell’s blunt declaration was taken as the starkest exhibition yet of the obstructionism that has characterized the Kentucky senator’s stance toward President Obama and congressional Democrats. The resistance from Mr. McConnell has had an enormous influence on the shape of Obama’s presidency. It has limited the president’s accomplishments and denied him the mantle of the postpartisan unifier he sought back in 2008. But it has also brought the Senate, the institution to which Mr. McConnell has devoted his life, close to rupture.