Syiem, 55, resigned as a member of the National Commission for Women today to contest the RS poll, party sources said here.
In the 60-member assembly, Syiem should get the backing of 43 MLAs of the ruling Meghalaya United Alliance -- Congress (29 members), NCP (2) and Independents (12).
The Opposition UDP and NPP are yet to announce their candidates for the by-elections.
The by-election was necessitated after Nationalist Congress Party member Thomas A Sangma resigned earlier this year to contest the February 23 Assembly polls from North Tura on a National People's Party (NPP) ticket. Sangma, however, lost the assembly poll.