Washington (CNN) -- It's the biggest question surrounding this year's midterm elections: how many people will turn out to vote?
The answer is crucial, because a smaller, more typical midterm electorate should favor the Republican Party. That's because single women, and younger and minority voters, who are big supporters of Democrats in presidential election years, tend to cast ballots in smaller numbers in the midterms.
That's the problem facing Democrats this November, as they try to hold onto their 55-45 majority in the Senate (53 Democrats and two independents who caucus with the party). The party is defending 21 of the 36 seats up this year, with half of those Democratic-held seats in red or purple states. In the House, the Democrats need to pick up a very challenging 17 Republican held seats to win back the majority from the GOP.
A CNN/ORC International poll released Monday illustrates the turnout dilemma for the Democrats.
In the generic ballot question, the Democrats have a two percentage point 47%-45% edge over the Republicans among registered voters. The generic ballot asks respondents to choose between a Democrat or Republican in their congressional district without identifying the candidates.
But when looking only at those who say they voted in the 2010 midterms – when the GOP won back the House thanks to a historic 63-seat pick up and narrowed the Democrats' control of the Senate – Republicans have a four-point 49%-45% edge.
"Younger Americans, women and non-whites score low on questions that ask them whether they are likely to vote. But on questions about how interested they are in the 2014 elections, women are not much different than men, non-whites are not that much different than whites, and people under 35 years old are not much different than people between 35 and 65 years of age," says CNN Polling Director Keating Holland. "The task for the Democrats is not to get those voters to pay attention to this year's election - the party faces the much more difficult task of turning that attention into enthusiasm for voting."
Two caveats: The margins are within the poll's sampling error. And the while the generic ballot is a much watched midterm indicator, the battle for Congress is not one race but instead 471 individual Senate and House contests across the country.
The poll was conducted for CNN by ORC International May 29-June 1, with 1,003 adults nationwide questioned by telephone. The survey's sampling error is plus or minus 3.5 percentage points for registered voters and plus or minus four percentage points for self-described 2010 voters.