The election of MK Shelly Yachimovich as the Labor Party's new leader is the first political expression of the wave of social protest that swept Israel this summer. Ever since that day about six years ago when she moved from the media into politics, Yachimovich has given voice to the social democratic idea and focused on it.

She insisted that dealing with social issues - the loss of solidarity, the enrichment of the few and the erosion of the middle class - had to precede a solution to the conflict with the Palestinians. For this, she suffered harsh criticism from the left, but she refused to deviate from her message. Now, she has reaped the political fruits.

Yachimovich now has a rare political opportunity to position herself at the forefront of the opposition to Benjamin Netanyahu's right-wing government. Kadima, headed by Tzipi Livni, has failed utterly to present an alternative to the prime minister's failed policies, and its Knesset members have supported racist legislation against the Arab community. The social protests exposed Livni's irrelevancy and cleared the way for another politician to unite the government's opponents behind him or her. This is Yachimovich's moment.

There's no doubt that Yachimovich has the skills needed to lead the opposition. She is a hard-working, sharp-tongued parliamentarian who attracts interest and provokes controversy. But this is not enough. For Labor under her leadership to be able to head the left in the next Knesset election, she will also have to give voice loudly, clearly and forcefully to the diplomatic message she has thus far avoided: that ensuring Israel's future requires a political separation from the Palestinians and the end of the settlement enterprise in the West Bank, with no preconditions. If she continues to blur her positions, Labor will look like a second Kadima and disappear for good.

In the run-up to the next Knesset election, Yachimovich will need to deal with the two weaknesses she has been accused of: lack of executive experience and difficulty in working with others. She must use this time to unite her divided, strife-torn party behind her and avoid fights with supporters of MK Amir Peretz, her former political patron and the man she defeated in the primary.

Her success in rebuilding the party from its ruins will be her first test as its leader and a candidate for high governmental office.