WASHINGTON -- U.S. Senate Democrats on Tuesday, for the second time in a week, blocked legislation meant to kill an Iran nuclear deal negotiated by the Obama administration and other world powers with Tehran.

By a vote of 56-42, the Republican-majority Senate fell short of the 60 votes needed in the 100-member chamber to advance the legislation.

The result was similar to last week's procedural vote on the Iran measure but two Republicans opposed to the deal, presidential candidates Rand Paul and Lindsey Graham, did not vote on Tuesday.

Senate Majority Leader Mitch McConnell failed in his attempt to lure at least a couple more Democrats into voting for the resolution of disapproval with this repeat of last Thursday's vote.

McConnell moved to arrange a third procedural Iran vote, possibly this week, on an amendment preventing Obama from lifting economic sanctions against Iran until that country's government formally recognizes Israel's right to exist and releases American citizens in Iranian custody.

It was not yet clear how Democratic votes would line up on that amendment.

(Reporting by Richard Cowan; Editing by Peter Cooney and Bill Trott)