Washington (CNN) For the second time in two weeks, Senate Democrats Tuesday blocked a Republican push for a final vote on the Iran nuclear deal, which was negotiated by President Barack Obama and is uniformly opposed by congressional Republicans.

The procedural vote, which needed support from 60 senators to pass, fell four votes shy: 56-42.

GOP Sens. Lindsey Graham and Rand Paul -- who both oppose the Iran deal -- missed the vote, thanks to Wednesday's CNN-hosted debate at the Ronald Reagan Library in Simi Valley California. The measure fell two votes short last week.

GOP leaders had hoped that two of those Democrats would buckle to pressure from constituents and flip their votes, but that was not the case.

The vote means the historic Iran deal, which lifts economic sanctions against Iran in return for it curbing its nuclear ambitions for the next several years, is set to be implemented despite majority votes in the House and Senate against it.

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