The president of the biggest opposition party on Monday dared Prime Minister Shinzo Abe to face him in a televised debate over the next two weeks leading up to the Upper House election, calling his refusal to do so unheard-of in recent years.

Democratic Party chief Katsuya Okada petitioned Abe to rethink his refusal to appear in TV debates with other party leaders during the campaign period ahead of the July 10 election.

"I have a suggestion for the prime minister," Okada wrote in a letter submitted to Abe. "Why don't you and I have an hour, or better yet, two hours of a fully public, thorough debate on the economy and the Constitution? I'll make myself available at any time."

Abe and other leaders participated in several debates hosted by major TV stations prior to the campaign kickoff last Wednesday, but as Abe has refused to participate in further debates, none will be held, Okada told reporters Friday.

Only one such event took place after the campaign kickoff, organized by TBS on Friday.

"This is unheard-of in recent years," Okada said in the letter.

When asked about his reluctance, Abe explained during a TV Asahi program on June 21 that he wanted to finish all public debates before the start last Thursday of early voting, which has become increasingly popular.

Indeed, an internal affairs ministry survey points to a spike in the number of people who vote early for Upper House elections, from 7.17 million in 2004 to 12.94 million in 2013.

That logic, however, does not satisfy Okada.

"There are loads of issues that we need to discuss. Unfortunately, our past discussions have come nowhere near being deep enough," he wrote. "I hope you will accept my proposal."