"We are expecting quite a bit of tension and internal disagreement among the close to 1,500 members when they meet to vote in Tel Aviv," she said.
"There has been so much opposition to entering into any agreement with Likud, because of the agenda of Likud and because of the previous commitment by Ehud Barak."
Barak had earlier pledged to stay in opposition if Labor won less than 20 seats in the Knesset, the Israeli parliament.
Labor, once the dominant force in Israeli politics, won only 13 seats in elections held on February 10.
In attempts to create a coalition, Netanyahu has already found allies in the nationalist Yisrael Beitenu party and the orthodox Jewish party Shas.
With Labor's support, Netanyahu would have 66 seats in the 120-seat Knesset - 27 from Likud, 13 from Labor, 15 from Yisrael Beitenu and 11 from Shas.
Many members of the centre-left Labor party oppose joining a cabinet led by the right-wing Netanyahu because of his limited interest in peace negotiations with the Palestinians.
Seven of Labor's 13 members of parliament had earlier sent a letter to Netanyahu saying: "We must inform you that you cannot count on our support regarding any agreement that you may reach with Ehud Barak."
Al Jazeera's Mike Hanna said Barak is acting unilaterally and against the wishes of most of Labor's MPs.
"What you're looking at is the possible implosion of the Labor party, the party which was so enormous in Israeli politics for so many years."
Hanna said Netanyahu is involved in a "gamble", as he cannot count on the support of a majority of the Labor MPs.
An agreement with Labor might also put Netanyahu's previous deals with the two right-wing parties at risk.
"He may no longer be able to pull in those parties on the right he needed to get an absolute majority.
"Netanyahu is on very dangerous ground and may not be able to present a government on the deadline, April 3," Hanna said.
Netanyahu has until April 3 to form a coalition.
If a deal is sealed between Labor and Likud, Netanyahu will commit to continuing negotiations with the Palestinians and to respect deals signed with them in the past, Israeli radio reported.
Labor would get five ministries in the new government. Barak is expected to keep his post as defence minister.
"There's a very important personal reason for this crusade. It's called Ehud Barak," Marwan Bishara, Al Jazeera's senior analyst, said.
"He doesn't want to be on the opposition because that will mean the end of his career for the time being," Bishara said.
Bishara said that if Labor did join the coalition, Barak would not be a senior decision-maker alongside Netanyahu and Avigdor Lieberman, the right-wing foreign minister-designate.