Kissing it all goodbye. (AP Photo/Pablo Martinez Monsivais)

John Boehner was elected House majority leader in 2006 after 15 years in Congress. And if you had told someone, at that time, that Boehner would resign his seat in less than 10 years for being insufficiently conservative, you probably would have been laughed at. But Boehner's career provides an illustration of the stunning rightward shift among House Republicans in recent years.

The chart above plots average conservative ideology scores among House Republicans from the 1960s to today. The data come from political scientists Keith Poole and Howard Rosenthal, who've created a widely-used ideology index called a dw-nominate score, which is based on individual legislators' voting records.

[Live coverage of Boehner's announcement]

When John Boehner first went to Washington in 1991, the average ideology score of House Republicans hovered somewhere between 0.3 and 0.4. But since then, that score has roughly doubled, indicating a strong shift rightward in House Republicans' voting patterns.

Boehner was obviously part of that shift -- after all, we're talking about the man who was the face of GOP opposition to Obamacare, shouting "Hell no you can't!" on the House floor. But he is also, fundamentally, a creature of the Republican party establishment, pragmatic enough to understand that you can't get your way all the time in a divided government.

For many in the younger generation of Republicans elected to the House in the tea party wave of 2010 and since then, this pragmatism was tantamount to surrender. Their distaste for the speaker grew so great that the chances of passing a clean spending bill this month with their support have grown since his announcement.

Ronald Reagan famously said that he didn't leave the Democratic party -- rather, the party left him. Today's resignation is a reminder that House Republicans are venturing into uncharted territory in their pursuit of ideological purity -- and leaving John Boehner behind.