If those predictions hold true, the Senate's political leanings would tip toward Mr. Aquino and away from Mr. Binay.

WSJ: Philippine Midterm Vote Sets Stage for 2016

If his election slate does better than expected, Mr. Binay will have shown his vote-getting prowess and fine-tuned his political machine.

WSJ: Philippine Midterm Vote Sets Stage for 2016

Mr. Binay is known for his personal touch, sending cakes to senior citizens on their birthdays, for example, and for his self-deprecating humor, such as joking about his 5 foot-2-inch height.

WSJ: Philippine Midterm Vote Sets Stage for 2016

Mr. Binay is looking beyond the Senate as well.

WSJ: Philippine Midterm Vote Sets Stage for 2016

"The coming election is definitely a referendum on President Aquino's first three years and will provide a glimpse of Vice President Binay's chances in 2016, " said Edmund Tayao, a political-science professor at the University of Santo Tomas.

WSJ: Philippine Midterm Vote Sets Stage for 2016

Greenberg and his group were joined by Vice President Jojo Binay, US Ambassador Harry K. Thomas, former Philam Group chairman Cesar Zalamea, ports king Ricky Razon, Union Bank president Justo Aboitiz Ortiz, Businessman Ronnie Concepcion, Philippines, Inc.

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Mr. Binay has much at stake, too.

WSJ: Philippine Midterm Vote Sets Stage for 2016

Mr. Aquino's Liberal Party-backed coalition slate of 12 candidates is expected to pick up nine Senate seats, while Mr. Binay's nine-candidate United Nationalist Alliance slate likely will take three seats, according to a survey by the nonpartisan Social Weather Stations conducted May 2-3.

WSJ: Philippine Midterm Vote Sets Stage for 2016