After Netanyahu's announcement, Yachimovich said there was a "reasonable probability" of winning.
"The public today understands that security is not just on the borders but is also job and income security and health and education security," she said.
Perhaps the most viable candidate to replace Netanyahu is former Prime Minister Ehud Olmert, but he is entangled in a legal battle that will keep him on the sidelines for the coming months.
Lagging behind Labor in the polls are Foreign Minister Avigdor Lieberman's ultranationalist Yisrael Beiteinu party, a new centrist party led by former TV anchorman Yair Lapid and the decimated Kadima Party, which is currently the largest group in parliament but has slipped badly in the polls under new leader Shaul Mofaz.
"I think the decision for early elections is a day of hope for the citizens of Israel," Mofaz told Chanel 10 TV. "It is an opportunity to replace the bad Netanyahu government that has isolated Israel politically over the past four years, damaged Israel's deterrence and deteriorated the middle class."
During the campaign, opponents are likely to seize upon Netanyahu's rocky relationship with Obama over how to handle Iran. The rift has unsettled relations with Israel's closest and most important ally.
Netanyahu could also come under fire for his failure to advance peace talks with the Palestinians, massive street protests in Israel last summer against the growing gap between rich and poor, and widespread resentment over attempts by ultra-Orthodox parties to impose their ways on general society.
Despite these shortcomings, Netanyahu remains popular in opinion polls, thanks to a lengthy period of quiet, a resilient economy and his handling of the Iran issue.
Peace talks with the Palestinians could also possibly be renewed after Palestinian President Mahmoud Abbas appeared to drop the main demand that has prevented talks.
Abbas told European diplomats that he would resume talks after the U.N. votes on a Palestinian request for "nonmember state" status, backing down from a previous demand to freeze all Israeli settlement activity before peace talks can resume. A vote is expected in November.
The official Palestinian Wafa news agency quoted Abbas as saying that once the U.N. membership is completed "we will be ready to return to the table of negotiations with the Israeli side to discuss all outstanding issues between us on the final status."
Abbas previously refused to meet Netanyahu while Israeli settlement construction was taking place in the West Bank, where he hopes to establish a future Palestinian state.
Abbas made no mention of a settlement freeze, and officials said Abbas believes a freeze would no longer be necessary if he receives U.N. recognition of a state that includes all of the West Bank.
Peace talks have been frozen for nearly four years, in part because of Palestinian demands for a settlement freeze and Israel's refusal to accept it.