Kumar's predecessor Jitan Ram Manjhi had taken trust vote three days after taking oath last year and Kumar himself had done so within five days of being sworn-in as chief minister in 2010, Modi said.
"But this time, Kumar would have to prove his majority on the floor of Assembly before March 16, more than three weeks," he said, adding that if the new CM was confident that his government enjoys majority support, he should have immediately taken trust vote.
It was surprising that the RJD supremo kept mum on the turn of political events in Bihar since he accompanied Kumar to parade 130 MLAs before the President, Modi said.
Actually, the BJP leader claimed, it was simmering trust deficit between the two leaders due to which Kumar was reluctant to take trust vote and expand his ministry.
Apart from this, there was no reason why the budget session of the legislature could not be convened soon as the Governor's address to joint sitting of the two houses as well as the budget of the JD(U) government led by Manjhai was ready, he said.
Modi said as far as he was concerned, the JD(U) is continuing in power as the same ministers constituted the governments headed by Kumar and Manjhi over the past 21 months and these ministers held same portfolios under the two.
The former deputy chief minister also criticised his erstwhile boss for apologising for resigning as chief minister after JD(U)'s drubbing in general elections last year.
Kumar should reply to a question whether the people are happy with him for joining hands with the RJD and Congress to run a minority JD(U) government, Modi said.
On the Chief Minister conceding that governance had suffered in Bihar over the past nine months since he demitted office, Modi said Kumar should be ready to take responsibility for deterioration in law and order and development coming to a halt as he committed a mistake by severing ties with the NDA.
The former deputy chief minister also slammed Kumar for taking credit for the NDA's landslide victory in 2010 Assembly polls, saying the mandate was for both JD(U) and BJP.
Modi expressed doubt about Kumar's promise to bring governance back on track over the next nine months before Assembly polls due later this year, as he has only six months before the model code of conduct comes into effect around first week of September.
The Chief Minister cannot deliver on development and law and order fronts so long as he survives on the crutches of the RJD and Congress, the senior BJP leader said. Modi also charged Kumar with being arrogant as he had yesterday said he may meet the Prime Minister to seek his support for development of Bihar if he felt it was necessary to do so.
Asked whether BJP will rope in Manjhi to boost NDA's electoral prospects in Bihar, Modi replied in a guarded manner that the former chief minister continues to be an unattached MLA of the JD(U).
The BJP had publicly expressed solidarity with Manjhi and decided to vote in his favour on the floor of the House during the trust vote on February 20. However, Manjhi resigned beforehand apprehending that he did not have the numbers to win.