Abbas aides maintain he can stay in office until 2010.
The debate is not just about fine print. The Hamas deadline coincides with President Bush's target date for at least the outline of an Israeli-Palestinian peace deal. Few expect Bush's Mideast peace initiative to bear fruit unless Abbas can restore his rule in Gaza -- an outcome that would probably require renewed deal making with Hamas.
Abbas has been in control of only the West Bank since Hamas seized Gaza by force a year ago, and he'll probably keep his truncated job no matter what Hamas decides. But failure to lead his people to independence, along with a decision by Hamas to withdraw its recognition, would further weaken an already unpopular president and cement the rift between Hamas and Abbas' pragmatic Fatah movement.
"If the year ends without a peace deal, as people were promised, then Abbas and the entire Palestinian Authority, which was established to bring about this solution, would come under a big question mark," said political analyst Ali Jerbawi.
The dispute stems from conflicting interpretations of legislation.
Abbas was elected president in January 2005, winning 62 percent of the vote in the West Bank and Gaza. A year later, Hamas defeated Fatah by a landslide in parliamentary elections. The Basic Law, a forerunner to a Palestinian constitution, says both president and parliament are elected to four-year terms.
Before Hamas was elected, parliament passed a law stipulating that future presidential and parliamentary elections be held simultaneously. However, the subsequent Hamas-controlled parliament never amended the Basic Law to include this new clause. The upshot: Fatah believes Abbas has the right to stay in office an extra year.
Until now, Hamas has not challenged Abbas' position as president -- even allowing him to negotiate peace with Israel.
But in recent days, Hamas officials have said publicly that they consider Abbas' term over in January. After that, they plan to install Deputy Parliament Speaker Ahmed Bahar as interim president for 60 days before calling presidential elections, said Faraj al-Ghoul, a Hamas legislator in Gaza.
"Hamas is determined to fulfill the law," added Fawzi Barhoum, a Hamas spokesman in Gaza.
Such elections would presumably only be held in Gaza, without Fatah participation, because Abbas would refuse to go along.
The Hamas threat may be part of an attempt by the group to pressure Abbas into negotiating a power-sharing deal. Abbas has repeatedly said he will not talk to Hamas until it gives up power in Gaza. Last month, he expressed a willingness to resume dialogue, though he has been vague about whether his preconditions remain.
Abbas aide Saeb Erekat said he is "not surprised" by Hamas' threats, saying they are "consistent with their coup" in Gaza. He said the election law is clear that presidential and legislative elections must be held together.
In recent weeks, Abbas aides have hinted that the Palestinian leader might resign by the fall if negotiations with Israel don't move forward. Since peace talks were launched at a U.S.-hosted conference last November, the sides have reported no breakthroughs.
Bush called Abbas last week, promising to become more engaged if asked. Secretary of State Condoleezza Rice is to host the two negotiating teams three times by the end of the year.
If there's no peace deal, it's unlikely Abbas would call elections anytime soon.
He'd have nothing to run on as a candidate, and he has no apparent successor who could easily defeat a Hamas candidate. As the default option, Abbas would remain in office but in a diminished role with continued Western support.