TEL AVIV, Israel — Gadi Eisenkot is as low-key as Benjamin Netanyahu is bombastic. He's a political novice up against the longest-serving, most politically savvy prime minister in Israel's history.And yet, on the eve of Tuesday's elections, Eisenkot poses the most serious threat to Netanyahu's hold on power in years.Why it matters: A victory by the Yashar party leader would upend Israeli politics and could reset the country's strained relationships abroad.