The Palestinian leader, Mahmoud Abbas, has received another boost in his campaign to replace Yasser Arafat.
The group will closely monitor Mr Abbas' performance
The militant al-Aqsa Martyrs' Brigades - the armed wing of the Fatah movement - has announced that it will back his candidacy for the January poll.
Mr Abbas is a moderate and there have been questions as to whether he might be opposed by Fatah's militants, says the BBC's Alan Johnston in Gaza.
But they seem to have been persuaded by jailed Fatah leader, Marwan Barghouti.
Barghouti announced on Friday he would not run in the Palestinian presidential election, and urged supporters to back Mr Abbas.
Sunday's announcement comes amid efforts to fend off a split in the Fatah political movement over who succeeds veteran leader Yasser Arafat, who died earlier this month.
'Not tough enough'
The al-Aqsa Martyrs' Brigades are divided into several factions, but came together to issue the joint statement in support of Mr Abbas.
Mr Abbas has been an outspoken opponent of the armed Palestinian uprising to which the al-Aqsa Brigades have been totally committed - and some militants fear that he will not be tough enough in negotiations with the Israelis, our correspondent says.
Mahmoud Abbas is seen as a moderate
But the militants of al-Aqsa will watch Mr Abbas closely and if they were to decide that he was giving too much away to the Israelis, he could face serious opposition from the militants which might split the Fatah movement badly, he says.
Mr Abbas was nominated to run as Fatah's only candidate at a Fatah Revolutionary Council meeting in the West Bank city of Ramallah on Thursday.
The former Palestinian prime minister, also known as Abu Mazen, has already replaced Mr Arafat as Palestine Liberation Organisation (PLO) chairman and is now favourite to succeed him as president.