From the story by Tracy Wilkinson in the LA Times:
The battle between law enforcement authorities and drug suspects has claimed more than 11,000 lives since he took office in late 2006.
In May 2006, then-President Vicente Fox ... backed down only under pressure from the Bush administration, which complained that decriminalization for even small amounts could increase use.
But with about two weeks to go before crucial mid-term elections in which his party is struggling to maintain control of Congress, Calderon cannot afford to be seen as bowing to the United States, analysts say.
Already under intense criticism for the drug-related violence, Calderon needs to maintain good relations with his nation's Congress, where much of the opposition voted in favor of the decriminalization bill.
And so, political observers say, he probably will sign it into law.