More than half the states have held primary contests this year, representing nearly two-thirds of the country's population. Nearly 14 million people have cast votes in Democratic primaries so far -- an increase of nearly 60 percent from four years ago. On the Republican side, 12.3 million people have cast votes, which is an increase of just under 20 percent.
In the Colorado primary last month, Democratic turnout nearly tripled, rising 197 percent from 2014 levels. Republican turnout, by comparison, was up by about 30 percent.
"I don't think you can look at that and not see that the intensity is on the Democratic side," says Dick Wadhams, a GOP consultant and former chair of the Colorado Republican Party. "We would sure be crowing about it if more Republicans were voting in primaries."
Colorado may be a special case. For the first time, unaffiliated voters -- who outnumber both Republicans and Democrats in the state -- were able to participate in primary elections. Still, a clear majority of independents chose to cast ballots on the Democratic side.
"Consistent with what we're seeing with special elections over the past year" -- in which Democrats have picked up numerous legislative seats previously held by the GOP -- "there's more motivation among Democrats than Republicans," says Seth Masket, a political scientist at the University of Denver. "Unless we have an unusually popular president, which Trump is not, the midterms are a chance for voters in the out-party to register their anger and frustration."
Still, Masket and other political scientists think it's too soon for Democrats to celebrate. Increased primary turnout, he says, "doesn't necessarily match up that well for general elections."
Studies show that people choose to vote in primaries for different reasons than in general elections. One older study found that the increased numbers of presidential primaries in the 1970s and 1980s actually lowered turnout in the fall by 5 percentage points.
"Presidential and state primaries divert resources away from the general election and reduce turnout among the peripheral electorate who are most dependent on a mobilization effort," according to the study.
And while turnout is up relative to 2014, it's worth remembering that turnout that year was the lowest for any election since World War II.
In California, Democratic turnout increased this year by more than 80 percent, while Republicans' numbers were up 45 percent. But overall turnout was still only 37 percent -- an improvement from 25 percent in 2014 but right around the average turnout for midterms there over the past quarter-century. Similarly in Maryland, Democratic turnout rose by 21 percent, compared to just 2 percent among Republicans. But the actual turnout rate for the state primary was an unimpressive 25 percent.
It's not as though all the numbers point to bad news for Republicans. In several states, the party has made voter registration gains since President Trump's election two years ago.
Over the past year in Kentucky, the GOP has added more than 34,000 registered voters, while the Democrats have lost more than 2,000. That's in keeping with trends in the state since Barack Obama became president. Democrats, once dominant, have dipped below 50 percent of registered Kentucky voters. Many longtime registered Democrats have been voting Republican without formally switching their allegiance. Now, a good number are taking advantage of the ease of online registration to change their affiliation.
"We have people who are still Democrats because granddaddy was a Democrat, and granddaddy was a Democrat because of FDR," says Tres Watson, communications director for the Kentucky Republican Party.
Primary turnout and registration numbers offer some clues as to which direction voters are heading. But they can't say definitively who will turn out -- or which party will come out on top -- in November.
"It's kind of like who's ahead at halftime," says Jeff Hays, the current Colorado GOP chair. "What can happen in the next four months matters more to me."
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