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The congressional review period began on Monday after the administration submitted the required documents to Congress.
Republicans have been highly critical of the deal and many Democrats have expressed concerns or made clear they are not yet ready to endorse the agreement.
But while Congress can vote to reject the pact, it will take a two-thirds majority in both the House and Senate to override a presidential veto of any attempt to derail the deal.
The $1.67 million that AIPAC spent so far this year is more than the group has ever spent on direct lobbying during a six-month period — at least in the last 16 years, according to lobbying records in the Senate Office of Public Records database, which date back to 1999.
An AIPAC spokesman declined to comment.
AIPAC has long maintained a lobbying presence in Washington, spending at least $2 million annually for the last several years. But the group’s spending in 2015 is on track to surpass that figure. And it is funneling resources to support Citizens for a Nuclear Free Iran, a new 501(c)4 group that is expected to spend around $20 million on advertising and campaigns in up to 40 states to get opponents of the Iran deal to pressure their representatives in Congress.
Correction: a previous version of this story incorrectly stated the name of the American Israel Public Affairs Committee. This version has been corrected.