Yesterday, Jim Webb, who was running for the Democratic Presidential nomination, dropped out of the race, and suggested that he may run as a third-party candidate. In his announcement, he blasted the two parties and suggested that the real political force in America is independents. “Our political candidates are being pulled to the extremes,” he said, at the National Press Club in Washington. “They are increasingly out of step with the people they are supposed to serve. Poll after poll shows that a strong plurality of Americans is neither Republican nor Democrat. Overwhelmingly they’re independents. Americans don’t like the extremes to which both parties have moved in recent years, and I don’t blame them.”

Today, Joe Biden announced that he would forgo a Presidential run, and he made a similar case that America is being damaged by excessive partisanship. “I believe that we have to end the divisive partisan politics that is ripping this country apart,” Biden said, in remarks in the Rose Garden. “And I think we can. It’s mean-spirited, it’s petty, and it’s gone on for much too long. I don’t believe, like some do, that it’s naïve to talk to Republicans. I don’t think we should look at Republicans as our enemies. They are our opposition. They’re not our enemies. And for the sake of the country, we have to work together. As the President has said many times, compromise is not a dirty word. But look at it this way, folks: How does this country function without consensus? How can we move forward without being able to arrive at consensus? Four more years of this kind of pitched battle may be more than this country can take. We have to change it. We have to change it.”

Finally, over in the House, as the majority party struggles to find a new Speaker, Congressman Charlie Dent, a moderate Republican from Pennsylvania, floated the intriguing idea of putting together a “bipartisan coalition” to elect the next leader. He called it “basic math”: if Republicans don’t have two hundred and eighteen votes for a Speaker, they will need to nominate a candidate that can win some votes from Democrats. If Paul Ryan, who laid out several conditions in order for him to formally enter the race for Speaker, decides against running—or if he is defeated—then perhaps Dent’s idea will gain currency. For now, it seems as dead as Biden and Webb’s candidacies.

The dilemmas faced by Webb, Biden, and Dent were different in many ways, but common to all three is a familiar structural fact of American politics that is making centrist politics seem increasingly out of reach and anachronistic. In 1950, the American Political Science Association published a major report on the state of America’s two-party system. The authors worried that the public had little idea what to expect when they voted for a Democrat or a Republican, because each party was a mish-mash of interests with little ideological consistency. What was needed in American politics were parties with more “internal cohesion,” so that when one party was in control the average voter would understand generally where it stood on the major issues. In addition, the party out of power needed to present sharp differences. The report said, “The fundamental requirement of accountability is a two-party system in which the opposition party acts as the critic of the party in power, developing, defining, and presenting the policy alternatives which are necessary for a true choice in reaching public decisions.”

The report had many recommendations about improving politics in America, but the basic headline was one that seems amusing today: America wasn’t polarized enough. Starting in the seventies, that all began to change. This graph from the Pew Research Center documents how the two parties, which were once both coalitions of liberals and conservatives, gradually began to pull apart until they were perfectly sorted: all liberals are now in the Democratic Party and all conservatives are now in the Republican Party. There is no longer any middle.

This great hollowing out of the political center helps to explain the week’s events. Webb’s analysis about independents being the greatest bloc of votes in America is wrong. Most self-described independents act like partisan Democrats or Republicans, but they simply don’t like to use the party labels. Webb will have no more success running as a third-party candidate than he did running as a Democrat.

Biden’s dilemma is that he seemed to want to be the nominee of his party without having to do the necessary work of wooing the partisans that dominate the process. In his speech, he indirectly criticized Hillary Clinton, who recently joked that Republicans are her enemy. It’s a fair criticism, but Clinton will likely be the nominee of her party because she has been doing the gruelling work of putting together the coalition of partisans who will decide the Democratic primary.

Over in the House, Dent’s proposal for a bipartisan Speaker has little chance of success, for similar reasons. The differences inside the G.O.P., where forty or so right-wing members are holding out support in exchange for concessions from the new Republican Speaker, are minor in comparison to the ideological differences between the two parties.

The center is dead in American politics. The candidates who thrive are the ones who understand that wooing and taming party partisans is the only path to victory.