The opposition candidate for the presidential election, Yashwant Sinha, reached out to Prime Minister Narendra Modi and Defence Minister Rajnath Singh on Friday, seeking their support for the July 18 polls.
Mr Sinha also dialled Jharkhand Chief Minister Hemant Soren and reminded him of the commitment that the Jharkhand Mukti Morcha (JMM) had made when he was named the combined opposition candidate for the presidential polls.
"We have begun our campaign in earnest and will reach out to everyone to seek their support in the election," sources in the Nationalist Congress Party (NCP) said.
They said Mr Sinha made calls to the offices of PM Modi and Mr Singh and left a message seeking support for his candidature.
The former Union Minister also reached out to his mentor and BJP leader LK Advani.
Mr Sinha is scheduled to file his nomination papers on Monday in the presence of top opposition leaders.
The JMM and the Janata Dal (Secular), led by former Prime Minister HD Deve Gowda, are being seen as supportive of the National Democratic Alliance's (NDA) presidential candidate, Droupadi Murmu, who filed her nomination papers on Friday.
Mr Sinha, who was expected to launch his campaign for the presidential election from his home state Jharkhand on Friday, was forced to delay it when it emerged that Mr Soren is leaning in favour of Ms Murmu, a fellow Santhal community member.
Meanwhile, Mr Sinha wrote a letter to all the opposition leaders who have chosen him as their common candidate for the July 18 polls.
"I assure you -- and the people of India -- that, if elected, I shall conscientiously uphold, without fear or favour, the basic values and guiding ideals of the Indian Constitution," Mr Sinha said.
He said he plans to launch his campaign after filing his nomination papers on Monday by visiting as many state capitals as possible.
"I hope to meet you as well as the MPs and MLAs belonging to your party to seek your support and guidance," Mr Sinha wrote in the letter.