Voter turnout in the US took a jump after (and probably because of) the civil right legislation post WWII and it's been on an upward trend again since about Obama - but it's still barely to 50% of the population. Given the US insistence on maintaining a relatively high level of inconvenience to vote, probably 25% of people who have some interest in voting end up failing to, so that plus the people who do vote gets you in the ballpark of 2/3 of the pop.
https://en.wikipedia.org/wiki/Voter_turnout_in_United_States_presidential_elections