Senate Democrats are demanding Majority Leader Mitch McConnell Addison (Mitch) Mitchell McConnellPelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Senate GOP aims to confirm Trump court pick by Oct. 29: report Trump argues full Supreme Court needed to settle potential election disputes MORE bring up a straight 10-year extension of key Iran sanctions once lawmakers return to Washington next month.

Seven Democrats — led by Sen. Richard Blumenthal (Conn.) — sent a letter to the Kentucky Republican asking that he "prioritize" a clean extension of the Iran Sanctions Act during the Senate's end-of-year session.

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"Passing this vital legislation before its expiration is crucial to ensuring with the utmost certainty that the United States will continue to have the sanctions enforcement mechanism our national security demands," they wrote in Wednesday's letter, a copy of which was also sent to Sen. Richard Shelby (R-Ala.) who chairs the Banking Committee.

McConnell said last year that any Iran proposal would need to have 67 votes — enough to overcome a potential veto — before he would allow it to get floor time.

The Iran Sanctions Act will expire at the end of the year without congressional action. Though there is wide-spread support for extending ISA, lawmakers are deeply divided over what should be included in an extension.

Republicans want to tie the ISA extension to broader penalties against Iran amid lingering fallout over last year's separate nuclear agreement with Iran, and a string of recent missile tests by that country.

Cardin told reporters last month that while he thinks lawmakers will be able to come to do a deal there's a "risk factor" to waiting until the lame-duck session to try to move the sanctions extension.