"We need to protect our national interests at all costs," Ishiba, who heads the ruling Liberal Democratic Party, said in the debate at the Japan National Press Club in Tokyo.
The debate was held before the start of Thursday of the official campaign period for the July 20 election for the House of Councillors, the upper chamber of parliament.
Asked about the possibility of his ruling coalition losing its Upper House majority, Ishiba said, "We'll take any results seriously and gravely."
The ruling bloc gave up its majority in the House of Representatives, the lower parliamentary chamber, in last year's election.
Yoshihiko Noda, leader of the main opposition Constitutional Democratic Party of Japan, indicated that his party will aim to oust the ruling coalition from power, depending on Ishiba's decision after the Upper House election.
As of Wednesday, about 520 people are expected to vie for 125 Upper House seats up for grabs.
[Copyright The Jiji Press, Ltd.]