President Megawati Sukarnoputri forged a pact with Indonesia's largest political group Golkar to form a bloc that will dominate parliament but is seen as unlikely to ensure her reelection.
Golkar, Megawati's Indonesian Democratic Party of Struggle and the smaller United Development Party and Christian-based Prosperous Peace Party, will now control 307 of 550 seats in Indonesia's House of Representatives.
They are the first parties to announce a formal coalition as lobbying intensified ahead of a September 20 presidential vote, in which Megawati faces a tough challenge from her former top security minister Susilo Bambang Yudhoyono.
The coalition united party leaders in defence of principles ranging from people's sovereignty to social welfare, human rights and citizens' equality.
"This coalition is to build the future. We no longer dwell on the past. We have to look forward," Golkar chairman Akbar Tanjung said after the four parties' leaders signed the agreement at a luxury Jakarta hotel.
Tanjung said Golkar, which won general elections in April, would back Megawati's party as they shared similar platforms and visions.
But Golkar's leadership freely admitted on Monday that its backing of Megawati was a display of realpolitik as party executives looked to cash in on "attractive" offers of ministerial posts and senior regional positions.
Megawati, meanwhile, hailed the deal, saying God willing, "victory will be in our hands." Analysts say the support of large parties for either candidate would probably not influence an increasingly independent-minded electorate, but will be needed later to create a stable government.
They also said support from Golkar, once the tool of former dictator Suharto's New Order regime and still seen as a retrogressive political force despite claims of change, could hurt Megawati's campaign pledges of reform.
Dewi Fortuna Anwar, of the Habibie Center think tank in Jakarta, said the coalition is trying to imitate the success of Malaysia's Barisan National, a ruling coalition which unites members behind similar social principles.
"This is interesting to watch because apart from the presence of the New Order forces, Mega is now the symbol of status quo while during the 1999 election, she was the symbol of reform," she said, adding the vote was still wide open.
"Our voters are a floating mass and they are now more educated. No one party in Indonesia, even Golkar which by far is the most organized party, has a well-established grip in the grass roots," she said.
In the initial round of Indonesia's first ever direct presidential election on July 5, Megawati won 27 percent of the vote, behind Yudhoyono's 33 percent.
Yudhoyono who has yet to form a coalition on Thursday denied he has ignored the role of political parties in the second round. "I founded a political party, the Democratic Party," he said. "What I say also is, don't fail to listen to the voice of the people."