JAKARTA (REUTERS) - Indonesia's most popular Islamic party on Saturday announced its support for presidential frontrunner Joko "Jokowi" Widodo in his bid for the top job in July, the third party in his growing coalition.

The National Awakening Party (PKB), which is aligned with Indonesia's largest Muslim organisation, is the latest party to join an alliance led by Jokowi's Indonesian Democratic-Party of Struggle (PDI-P).

The coalition also includes the National Democrat (NasDem) party and should help give Jokowi a broad base of support to help him pass major reforms if elected.

"PKB will join the coalition led by PDI-P...because we believe their vision to continue the political and economic development of Indonesia is similar to ours," said Mr Helmy Faishal, senior PKB official.

PDI-P's Deputy Secretary General Hasto Kristiyanto confirmed the coalition with PKB, saying the two parties had "similar historical traces and ideology".

PKB was founded by former president Abdurrahman Wahid, known as Gus Dur, long-time head of the moderate Muslim organisation Nahdatul Ulama (NU) and whose administration in the early 2000s took official steps to embrace and protect various religions.

Overt displays of religion, including Islam, were repressed for years under the rule of former authoritarian leader Suharto.

The party, which calls itself a nationalist party, repeated Gus Dur's strategy this year by recruiting non-Muslim members to drum up support beyond its traditional rural, Islamic voter base.

Final results from Indonesia's legislative election confirmed on Friday that PDI-P had won the most votes, but failed to get enough to nominate Jokowi for the presidency without a coalition.

With the support of NasDem, however, PDI-P had enough to meet that threshold, and with the additional backing from PKB it will be even more difficult for Jokowi's main rivals to build the coalitions necessary to run for president.

Former general Prabowo Subianto, who heads the Gerindra party and is running a distant second in opinion polls, has yet to form a coalition with another political party.

Gerindra party officials have been in talks with Golkar, once the parliamentary vehicle of the long-serving Suharto, as well as others.