A new Washington Post-ABC News poll offers fresh evidence that Democrats are facing major enthusiasm problems within their base that make it difficult -- if not impossible -- for them to rebuild the winning coalition put together by President Obama in 2012.

While nearly seven in 10 of all registered voters say they are "absolutely certain" to vote in November, several key Democratic constituencies are much less committed to voting. Barely half of voters ages 18 to 39 are certain about voting (53 percent) and 55 percent of non-whites describe themselves as certain to cast a ballot. By contrast, more than seven in 10 whites and voters older than 40 say they will definitely cast ballots -- both groups that have favored Republicans in the past two elections.

The turnout gap is smaller among self-identified partisans, with Democrats six percentage points less apt than Republicans to be certain voters (72 percent vs. 78 percent). Closing that gap, however, could be difficult, given that Democrats are more than twice as apt to rate themselves "50-50" or less likely to vote; 15 percent of Democrats say this, compared with 5 percent of Republicans.

Beyond core partisans, independents who tilt Democratic are strikingly less motivated. Nearly three-quarters of independents who lean Republican are "certain" voters (74 percent), compared with just 50 percent of Democrats.

The dynamic parallels Republicans' turnout advantage in 2010. In that election, turnout dropped sharpest among young voters, African Americans and Hispanics. Those who showed up supported Republican House candidates over Democrats by 51 to 45 percent, despite Democrats holding leads among polls of registered voters. In the final Post-ABC pre-election poll, Democrats led by five points among all voters but trailed by four points among likely voters -- a nine-point swing driven entirely by voting at higher rates.

Republicans may not even need a 2010-like turnout wave to repeat the same popular vote victory margin. This is because Republicans are performing better among registered voters overall -- a five-point deficit in October 2010 stands at just one point today (45 percent support Democrats, 44 percent Republicans). Among "certain" voters, the current poll gives Republicans a 49 to 44 percent advantage, implying a smaller turnout-driven shift than four years ago.

The findings demand caveats, including the fact that poll respondents routinely overstate their interest in voting and even self-assessments are easier to make closer to the election. In addition, most voters have yet to tune in deeply to candidates or campaigns, or encounter voter mobilization efforts that could persuade them to vote.

All that said, 2014 is starting to look a lot like the 2010 election. And that's a very good thing if you are a Republican.

Peyton M. Craighill contributed to this report.