Prime Minister Shinzo Abe is riding a popularity wave unseen by six immediate predecessors, including himself in his first short-lived stint, as he pushes his Bank of Japan nominees through a divided Diet, raising the odds of the ruling party winning a July election.

Buoyed by a surge in the benchmark stock index to a 4½-year high — spurred by his antideflation policy — Abe has the highest approval ratings since the popular Junichiro Koizumi stepped down in 2006 and he was installed without a general election. This week he may secure his biggest achievement since he took the helm in December, with the Democratic Party of Japan, now the largest opposition force, signaling it will back Haruhiko Kuroda for BOJ chief.

With analysts from Citigroup Inc. to Barclays PLC raising growth forecasts, an improving economic outlook gives the ruling Liberal Democratic Party a tailwind heading into the July election for the Upper House, which it doesn't control. A win may bring an end to an annual revolving-door of leaders, and give Abe a mandate to pursue free-trade talks opposed by farmers.