By Dave McKinney

CHICAGO (Reuters) - A group aligned with a conservative Illinois think tank sued the state in federal court on Thursday, challenging a recent law allowing Election Day voter registration at polling places in the state's most populous counties.

The statute in question, passed in late 2014 by the Democratic-led legislature and enacted in early 2015 by former Democratic Governor Pat Quinn, for the first time allowed Election Day voter registration, including at polling places.

But the section of the law pertaining to polling place registration applied only to counties with populations of 100,000 or more.

The Chicago-based Liberty Justice Center believes that population threshold unconstitutionally discriminates against voters in Illinois’ less populated counties and also gives a boost to Democrats in heavily Democratic Cook County, home to Chicago and birthplace of Democratic presidential nominee Hillary Clinton.

“It hardly seems coincidental Democratic candidates tend to do better in high-population counties, and Republican candidates do better in low-population counties,” said Jacob H. Huebert, senior attorney for the group.

Huebert said his group will seek an injunction against polling-place registration for the Nov. 8 election.

Huebert’s organization was founded by the Illinois Policy Institute, which keeps its donors secret. Republican Governor Bruce Rauner publicly has acknowledged giving the Illinois Policy Institute at least $500,000 over a period of years.

State Senator Don Harmon, a Democrat from suburban Chicago and the law’s chief Senate sponsor, said Illinois law commonly differentiates between counties' populations and said election officials in less-populous counties voiced concern about not having resources for polling place registration.

Under the law in question, voters in smaller counties can still register on Election Day in county clerks' offices.

Harmon suggested the lawsuit’s real aim is to dampen Democratic voter turnout this fall, particularly in party strongholds like Cook County. In 2015, there were 4 million residents in the county above the voter-eligible age of 18, according to the U.S. Census.

In the March presidential primary, 1.4 million votes were cast from Cook County, nearly 80 percent of which were Democratic, state records show.

“I suspect the plaintiffs are much more interested in having same-day registration thrown out in Cook County than they are in extending it to every small county in Illinois,” Harmon said.

Thirteen states, including Illinois, allow qualified residents to go to the polls or an election official’s office to register before or on Election Day, according to the National Conference of State Legislatures.

(Reporting by Dave McKinney; Editing by Tom Brown)