In Oregon, a quorum is two-thirds of the legislative body — 40 out of 60 representatives or 20 out of 30 senators. Is there some governing rationale for this higher requirement? Not really. It’s just something the fledgling state of Oregon copied from Indiana when it was assembling its constitution.
As of 2018, Democrats have supermajorities in both houses of the state legislature, with 18 out of 30 Senate seats and 38 out of 60 House seats. This needs emphasizing, since much of the media coverage of this story bizarrely omits it: Democrats represent a large majority of Oregon voters. They have much more public support than Republicans in the state.
oregon legislature
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However, even given their small minority, if every Republican chooses to walk out of the Senate — literally refuses to show up and do their job — they can prevent a quorum, thus preventing any legislative business from being done in the Oregon legislature. (It’s especially effective in even-numbered years, when the session is only 35 days long.)
Of course, either party could have done this at any time in Oregon history when it was in the minority. They just didn’t. It was commonly understood, without needing to be stated, that walking out on the job would be a gross dereliction of duty and an insult to Oregon voters. If both parties made a practice of it, governance in the state would become completely impossible.