Factbox: U.S. Senate Democrats line up behind Iran nuclear deal

Sep 2 (Reuters) - Another U.S. senator declared support on Wednesday for the U.S.-led nuclear agreement with Iran, providing a crucial 34th vote to protect it from being killed by Republicans in Congress. Democrat Barbara Mikulski provided the pivotal commitment to defending the deal negotiated by the United States and other world powers. The pact would put new limits on Iran's nuclear program while lifting sanctions on the country. President Barack Obama needs the backing of 34 senators to ensure lawmakers cannot override a likely veto by him of a measure to disapprove the agreement. Counting Mikulski, 32 Democratic senators and two independents who vote with Democrats have pledged support for the deal. The following describes how votes are likely to play out: * When Congress returns on Sept. 8 from its long Augustrecess, debate will begin on a Republican-sponsored "resolutionof disapproval" against the deal. * In the Senate, the Republicans must gather 60 votes tomove the resolution forward under Senate procedural rules. Ifthey can, they will then need 51 votes to approve theresolution. They have until Sept. 17 to get this done. * There is no similar procedural barrier in the House. Theresolution is expected to easily win approval there. * If both chambers approve the resolution, it would go toObama's desk for review. He has vowed to veto it. * If he does, opponents would try to override the veto. Thiswould take a two-thirds majority in each chamber. The Senate has100 members; the House, 434, plus one vacant seat. * Democrats could block an override in the Senate with 34votes. So far, 34 senators have committed to back the deal. * In the House, if Republicans voted unanimously against thedeal, they would need to get at least 44 Democrats to vote withthem to override a veto. * The Iran deal is not a treaty, so it does not need atwo-thirds vote in the Senate. The "resolution of disapproval"mechanism was part of a law Obama signed in May to give Congressthe right to weigh in on the Iran deal. * If Congress were to pass a resolution of disapproval andoverride a veto, Obama would be barred from temporarily waivingmost of the U.S. sanctions imposed on Iran over its nuclearprogram. Proponents of the agreement argue that this would killthe deal. (Reporting by Patricia Zengerle, Susan Cornwell, Susan Heavey and Alex Wilts; Editing by Kevin Drawbaugh and Lisa Von Ahn)