Dr Mahathir Mohamad, who helmed Umno for more than two decades, has penned an open letter urging grassroots leaders to reject the leadership of Prime Minister Najib Abdul Razak.
Mahathir's first open letter against a prime minister was in 1969, addressed to first premier Tuanku Abdul Rahman.
With the possibility of BN defeat looming today, he said Umno grassroots must reject Najib before the next general election.
"Reject his leadership before GE14 so that Umno would be safe and be able to win the election to remain as a fair government to all," he added in the letter posted on his blog.
The former premier said he resorted to writing on his blog because of the efforts to prevent him from meeting Umno members and the public.
"The machinery used (to block him) consisting of government and party bodies is most effective. This is in itself is proof of the wrongs being committed," he added.
Umno is slated to hold its annual general assembly from Dec 8 to 12.
Delving into history, Mahathir said Malaysia was born out of Umno's struggle as a party, and not because of an individual.
However, the former premier lamented that now loyalty appeared to be with the president, and not Umno, race or religion.
"Supporting Najib is perhaps rewarding for certain people. But when BN loses (in the election), all this would be lost.
"There is a big possibility the party would lose in GE14 like in Selangor and Penang (in the previous election).
"BN and Umno would then become an opposition party. As an opposition party, Umno cannot do anything for the race, country and religion," he said.
On the contrary, Mahathir said if Najib quits as Umno president and prime minister, there is a huge probability of Umno recovering and BN winning the next election.
"With that (a victory), race, country and religion would be much safer. The country would once again develop and the people would benefit," he added.
Exhorting Umno grassroots leaders and members to think, Mahathir cautioned them against blind loyalty or support because of the rewards garnered at present.
"Our race would condemn us if their fortunes dwindle (in the future) because we were greedy and forgot the Umno struggle.
"All that we have received would be destroyed," he said, warning that if political rivals formed the government, they might act on the wrongs committed before.
"If you wish to be safe and win the next polls, reject Najib's leadership before the next election," he added.