This post was updated at 5 p.m. ET Thursday

Congress is back with a daunting must-do list, but its first order of business was the Iran nuclear deal. Senate Democrats successfully filibustered a disapproval resolution on Thursday that was meant to kill the deal. The vote was 58-42 --Senate Republicans and other opponents needed 60 to pass.

It's an unusual vote (as NPR's Steve Inskeep explained) — instead of voting on whether to approve the Iran deal, opponents tried to bring forward a disapproval resolution.

Here's a look at how the Iran vote is different and why it's not quite business as usual on Capitol Hill:

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