Julian Zelizer is a history and public affairs professor at Princeton University, editor of "The Presidency of Barack Obama: A First Historical Assessment" and co-host of the "Politics & Polls" podcast. Follow him on Twitter: @julianzelizer. The opinions expressed in this commentary are his own.

(CNN) During an appearance in Michigan, former House Speaker John Boehner spoke ruefully about the state of his party. "There is no Republican Party. There's a Trump Party," he said. "The Republican Party is kind of taking a nap somewhere."

Unfortunately for Boehner, it is not that easy for the GOP to avoid culpability. The former Republican leader can't simply absolve his party of responsibility for what takes place in the Oval Office. After all, President Trump is a Republican, not a Democrat. He made his way to the highest levels of power through Boehner's party.

Over the years, moreover, the GOP opened the door to this kind of politics — the disregard for norms of governance, the adversarial posture toward institutions, and the willingness to throw into the public conversation arguments that are untrue or tied to extremist elements of the political spectrum. And congressional Republicans, who have immense power, have done almost nothing to push back against the President when he aggressively flexes his muscles.

Boehner was there at the moment when the tea party came to town. The new generation of Republicans who arrived in Washington during Barack Obama's presidency were a rambunctious bunch. They were willing to take draconian steps to achieve their goals, even threatening to send the nation into default over spending disputes with the administration. The tea party used Fox News as a major platform to spread its narrative. Many Republicans were comfortable being part of conversations such as the Birther movement, which relied on falsehoods, to score political points.

When he was speaker of the House, Boehner made a strategic decision to work with the tea party, recognizing that it was a powerful bloc of votes within the GOP conference. He invited its members into the halls of power as a way to strengthen the party. "Garbage men get used to the smell of bad garbage," he explained upon his retirement.

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