Siddaramaiah, who was leader of the opposition in the outgoing assembly, and central Labour Minister Mallikarjun Kharge remained front runners for the post, according to Congress sources.
The Congress Wednesday scored a clear victory in the assembly poll winning 121 seats in the 225-member house.
The newly elected legislators have been called for a meeting at the party office here for discussion with the central observers, state Congress chief G. Parameshwara said in a press statement.
The central observers would be led by Defence Minister A. K. Antony and would include former central minister Ambika Soni and Congress general secretary in-charge of Karnataka Madhusudan Mistry, party sources said.
They would ascertain from the legislators who is their choice for chief minister and brief Congress president Sonia Gandhi who will take the final decision.
On Thursday two former ministers who also held top posts in the state Congress unit - R. V. Deshpande and D. K. Shivakumar - said their services to the party should be taken into account while picking up the chief minister.
Shivakumar, a former working president of the state unit, tried to play his caste card also saying he comes from a community, Vokkaligas, which has contributed much to the growth of the Congress in the state.
He told reporters here that he has won the assembly election for the sixth time in a row and has experience and ability to carry various sections of the party with him.
Deshpande, a former state unit chief, was not as direct as Shivakumar to declare that his name be considered for the chief minister's post.
"If the party gives the responsibility I will carry it out," he told reporters in Bangalore.
Earlier in the day, pitching himself as the next chief minister of Karnataka, Siddaramaiah said that all 120 of the party's newly elected legislators were with him.
"The observers will brief the party high command (on legislators' choice) which will take the decision," he told reporters at his residence here.
On the support he has among the legislators to become the chief minister, he said: "All 120 MLAs (members of the legislative assembly) are with me."
"I am a strong contender," Siddaramaiah had said Wednesday even as vote count was on and trends showed that the Congress was heading for a clear majority in the house.
Kharge had said Wednesday that if the high command thinks he was fit for the post, it was for it to take the decision. However, he had said he did not want to be considered because he is a Dalit.