Six million Americans didn’t cast ballots in the 2008 election because they missed a registration deadline or didn’t know how to register, according to organizers of Tuesday’s National Voter Registration Day. The national registration drive is a coordinated effort that combines field events, technology and media campaigns with the goal of reaching tens of thousands of eligible voters whom activists said they could not reach otherwise.

As he commemorated the 50th anniversary of the Voting Rights Act, which prevents states from creating racial and socioeconomic barriers to the polls, President Barack Obama last month proclaimed Sept. 22 as a day for Americans to make sure they are registered. Some voter advocacy groups have said this year’s registration drive is crucial to ensure historically disenfranchised and underserved populations speak with a louder voice in next year’s presidential election.

The Voter Participation Center, a Washington group, has coined the term “Rising American Electorate” to represent those underrepresented voters. Unmarried women, people of color and millennials are included in that group.

There are about 125 million eligible Rising American Electorate voters, according to the Voter Participation Center. That's approximately 57 percent of the nation’s vote-eligible population. Roughly 51 percent of millennials and Asians are not registered to vote, according to the research. Latinos are unregistered at a rate of 49 percent, unmarried women at 40 percent and blacks at 37 percent.

Overall, 42 percent of historically disenfranchised voting groups did not register to vote last year, compared with 26 percent of other Americans, according to the Voter Participation Center. As conservative politicians moved to enact voting laws that disproportionately affect underserved Americans, activists sounded the alarm.

“We should be mindful of the dark days in America when it was dangerous, and in some cases impossible, for women and people of color to register,” Page Gardner, president of the Voter Participation Center, wrote this week for the Huffington Post.