JEFFERSON CITY (AP) – Early voting has become an increasingly common convenience in many states. Yet it’s also becoming increasingly contentious for political parties.

In several key swing states, Republicans are attempting to curtail early voting by limiting the days on which is its available. Yet in other Democratic-controlled states, lawmakers have passed measures to expanding early voting.

The parties are trying to gain even the slightest electoral advantage for this year’s elections and the 2016 presidential campaign.

Although new research disputes it, there’s a general assumption that early voting aids Democrats more than Republicans. That’s partly because President Barack Obama used early voting to drive a greater number of Democrats to the polls in his 2008 and 2012 victories.

At least 33 states now have early voting laws.