Higher voter turnout tends to help Dems. Lower voter turnout tends to help the GOP.
The GOP has made a strategic decision to keep people from voting, by using typos, a middle initial, an added or missing apartment number in the Social Security and Motor Vehicle databases.
300,000 voters were barred from voting in Ohio in 2004. Here is a brief background on the current Ohio situation.
Interestingly, since Jan, 2007, over 600,000 new registered voters have happened.