WASHINGTON — Voting in midterm elections, which will determine control of Congress, ends this fall. But it starts in North Carolina this week.

On Friday, election officials will begin mailing absentee ballots there, followed soon by Alaska and Georgia — with no excuses required. Iowans can vote in person beginning Sept. 25. After decades of expansion in American voting methods, an estimated one-third of all ballots will be cast before the traditional Election Day on Nov. 4.

Yet this year, the trend collides with a Republican-led pushback in some states — for reasons of cost-cutting and election integrity or, as the Obama administration and civil rights groups suggest, crimping turnout by Democrats. Various new restrictions on voting, which range from more stringent identification requirements to fewer registration opportunities to curbs on early voting, have been put in place. A critical election variable is whether the new limits will tilt close races.

They might not. New voting restrictions have proven to be mobilizing tools for constituencies that feel threatened by them. In 2012, President Obama won battleground states, such as Florida and New Hampshire, where new limits had taken effect.