There has been an absolute decline in voters aged between 20 and 39, compared with 2018, and there has been a huge increase in voters aged over 50.
The notification will be issued on January 20, while January 27 will be the last date of filing nominations.
"Without the knowledge of the prime minister, is it possible to do this," Kumaraswamy asked, saying Modi should come clean on the issue.
The results for the bypolls in Uttar Pradesh, Madhya Pradesh, Karnataka, Bihar, Telangana, Maharashtra, Punjab and Tripura were announced on Tuesday.