Former Israeli army chief and rising political star Benny Gantz and centrist leader Yair Lapid have upended Israel’s upcoming elections with their decision today to join forces against Prime Minister Benjamin Netanyahu.

Why it matters: Netanyahu and his right-wing allies were favored to win on a security platform in the April elections. But Gantz’s “Blue and White” alliance with Lapid’s centrist Yesh Atid party and fellow former army chief Gabi Ashkenazi represents a major challenge. Gantz’s list is full of hawkish politicians, among them Moshe Yaalon, another former army chief and former minister of defense under Netanyahu.

Polls show the joint party has a good chance of winning more Knesset seats than Netanyahu’s Likud, an important symbolic victory. But the complexity of building a coalition with a majority of 61 Knesset members to form a government remains a daunting challenge.

The backstory: On Tuesday, Gantz called on Lapid to join him. After two days of negotiations, the pair struck a deal to rotate at the head of the government should they win, with Gantz serving the term of two and a half years.