The delays engendered in large part by the extended negotiations with Republicans in the Gang of Six, meant in the end that the normal legislative process—in which separate bills passed by House and Senate would be reconciled in a conference committee—was not going to work in this case. When the vacancy caused by the death of Senator Edward M. Kennedy was filled via a January 2010 special election by Republican Scott Brown, Democrats lost their 60th vote—and the McConnell strategy meant that there was no way, no matter what changes Democrats were willing to make in the final package, that there would be a single Republican vote to get them past the filibuster hurdle. Hence, the fallback to using reconciliation to bypass the filibuster in the Senate, and the inability to smooth out the rough edges and awkward language in the final bill that was enacted.
https://www.theatlantic.com/politics/archive/2015/07/the-real-story-of-obamacares-birth/397742/
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