Karroubi, who has twice been speaker of Iran's parliament, is the first person to be nominated to challenge Ahmadinejad.
The National Confidence Party unanimously approved Karroubi to stand as its candidate in the election, says party spokesman Ismail Gerami-Moqaddam.
Karroubi will consider whether to accept the nomination over the next few weeks, said Gerami-Moqaddam.
Former president Mohammad Khatami is also being pressed to stand for election in 2009 by other reformist parties.
The reformists won a minority of the 290 seats in Majlis (parliament) elections earlier in 2008 after the majority of their candidates were prevented from standing by the Guardian Council.
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