The red-white-and-blue logo of President Susilo Bambang Yudhoyono's Democratic Party made a surprise appearance on stage with presidential candidate Prabowo Subianto at a rally in Medan.
Some people in the crowd also waved Democrat flags and cheered as Mr Prabowo, the patron of the Great Indonesia Movement Party (Gerindra), thanked friends from the Democrats for backing him at the rally on Wednesday.
As the month-long presidential campaign enters its second week, Democrat leaders from across the country are signalling their support for Mr Prabowo and his running mate Mr Hatta Rajasa, although the party has not formally endorsed the pair and questions remain over whether it will do so.
While the Democrats came in fourth position with 10 per cent of the vote at the April general election, the party's endorsement of a ticket could prove critical to undecided voters, especially as the Prabowo-Hatta ticket is closing the gap with front runner Joko Widodo, or Jokowi, of the Indonesian Democratic Party-Struggle (PDI-P), and his running mate Jusuf Kalla.
"All Democrat cadres have been instructed to explain to supporters that the programmes of a Prabowo-Hatta administration are in line with those of the Democratic Party," Democratic Party deputy secretary-general Ramadhan Pohan told The Straits Times.
"We want a president who is independent, has authority and who can represent Indonesia well at home and on the international stage," he added.
Dr Yudhoyono, who is chairman of the Democratic Party and Mr Prabowo's senior at the military academy, has said he will remain neutral and not back either of the two tickets in the July 9 election.
But executive chairman Syarief Hasan said being neutral does not mean abstaining from voting, and party leaders will study the platforms of both sides and are free to back either side.
As a result, many have openly declared their support for the Prabowo-Hatta ticket, including Dr Yudhoyono's brother-in-law, former army chief Pramono Edhie Wibowo.