The 2,500 member assembly is expected to announce its decision today.
The pact contains painful concessions such as immunity for US forces from Afghan law and allowing them to enter Afghan homes if an American life is under direct threat.
“Whatever the Jirga tells him, whether they tell him to sign it before election or after the election, he will follow through,” said Hasseeb Humayun, a member of the group.
If the US pulls out its troops, other countries in the NATO alliance underpinning Karzai’s administration are expected to follow suit and a thinner international presence could deter donors from releasing promised funds.
Afghanistan remains largely dependent on foreign aid.