Looking for yet another sign Democrats are in trouble? Take note of who is showing up to vote in the primaries.

For the first time since 1930, Republican votes for statewide offices are outnumbering Democratic votes, according to an analysis from American University's Center for the Study of the American Electorate. And Republicans, eager to campaign against Democrats who control the House, Senate and White House, are casting primary ballots at the highest rate since 1970.

"It isn't surprising that Republican turnout increased," said Curtis Gans, director of the center, citing intense battles within the party and the opportunity for GOP gains in Congress and in governors' offices. "But what's likely to prove telling is the lower participation of the Democrats, the first...