Mandatory voting.

Some think it would make little difference but the difference would be pro-Democratic.

Obama suggests making it mandatory to vote.

Those who think the difference would matter also think it would be pro-Democratic.

Hence liberals favor it (along with votes for 16 year olds, convicts, people too lazy to get photo ID, resident aliens both legal and not, and maybe even foreigners) while conservatives oppose it (along with all those other things).