Former Japanese Defense Minister Shigeru Ishiba declared his candidacy Friday for the ruling Liberal Democratic Party's leadership race in September, in what is expected to be a two-horse race with Prime Minister Shinzo Abe.

"I'd like to realize politics that are honest, fair, humble and polite," Ishiba said in a press conference, in an apparent attempt to differentiate himself from Abe who has been haunted by persistent favoritism allegations related to a pair of school building projects carried out by people with ties to him or his wife.

The 61-year-old veteran lower house member, who has been outspoken about Abe's government and his policies, is making his third attempt to secure the top party post. He faced off with Abe in 2012, only to be defeated.

Ishiba also said it is important to fill in widening economic gaps between regions and generations, and for that purpose vowed to develop Tokyo and other areas altogether.

The LDP presidential election, which effectively decides Japan's next prime minister, will likely be held Sept. 20, with official campaigning starting Sept. 7.

Abe has yet to officially throw his hat in the ring, but he is certain to seek to win his third three-year term as LDP president and become Japan's longest-serving prime minister. If re-elected, he is likely to pursue his long-cherished goal to amend the Constitution, drafted by the U.S.-led occupation forces after World War II.

Ishiba, who has also served in other key posts such as LDP secretary general and minister in charge of revitalizing local economies, is likely to face an uphill battle as five of the seven LDP factions have already expressed their support for the incumbent.

But Abe, aiming to achieve an overwhelming victory to maintain his grip on power, is wary of how many votes Ishiba will secure from rank-and-file party members, many of whom are living in rural areas.

Although Ishiba secured the strongest support among them in the first round of voting in the previous contest, Abe garnered more backing in the runoff in which only Diet members voted.

In the press conference, Ishiba said revisions to Japan's supreme law will be one of the major issues in the upcoming election.

He has openly criticized Abe's plan to revise the pacifist Constitution by simply adding an explicit reference to the Self-Defense Forces, saying the change should be more drastic.

Ishiba, known as a security hawk, came up with his own constitutional revision proposal in February that drastically rewrote the war-renouncing Article 9, in a way that implies removing the current postwar constraints imposed on the SDF activities.

On the economic front, Ishiba has expressed appreciation for the "Abenomics" policy package including aggressive monetary easing by the Bank of Japan, but has also called for more measures to prop up the regional economy.

Ishiba has also proposed setting up a new ministry in charge of disaster prevention to better deal with natural disasters hitting the country.

In the LDP election, the contenders will seek to secure a majority from among the 405 votes by legislators and the same number by rank-and-file members. If no one wins a majority in the initial stage, a runoff will be held in which ballots by Diet members weigh more.

Another possible candidate, Internal Affairs and Communications Minister Seiko Noda, is struggling to garner support from 20 LDP lawmakers necessary to register her candidacy.

Abe is likely to make an official announcement about entering the race later in the month, sources close to him have said.