In the next 75 days, hundreds of millions of dollars will be spent to persuade an evaporating oasis of swing voters to support Mitt Romney or President Obama.

But precisely who is this rare species of electoral wonder -- the elusive swing voter?

It's a lot of independents, of course, but that's not the whole story. In fact, there is plenty of reason to believe the 2012 election is both about persuading the few independents voters who haven't made up their minds and about turning out the parties' bases.

Washington Post-Kaiser Family Foundation poll finds independents are among the most likely to say they are still trying to pick a candidate -- no surprise here -- but so are cross-pressured partisans who are out of step with their own party on key issues.

Overall, the survey found 20 percent of Americans either cannot choose between Obama and Romney or say there's a good chance they will change their mind by Election Day. But the survey also found an extraordinary range of swing voters across the 13 different groups of partisans and independents -- as little as 5 percent in one group and as much as 58 percent in another.

(The groups were identified using a statistical technique known as "cluster analysis," that organized Republicans, Democrats and independents according to their opinions on a wide range of social and political values. See this graphic explaining the traits each group identified in the survey).

Independents classified as 'deliberators' were far and away the most persuadable voters: Nearly six in 10 had no current preference for Obama or Romney or said there was a good chance they'll change their mind. The lack of certainty is understandable given that 75 percent of deliberators say neither party is better than the other on the economy, the No. 1 election issue.

Two Republican groups stick out for an outsized number of voters on the edge. "Pro-government" and "window shopper" Republicans both believe government should do everything possible to improve living standards, marking a break with others in the GOP. "Window shoppers" -- the youngest Republican group -- also support legal same-sex marriage and abortion. While majorities of each group currently supports Romney, more than a quarter are currently undecided or say there's a good chance they could change their mind.

"Urban liberal" Democrats and "tea party" Republicans are the most certain in their voting decisions; fewer than 7 percent in each group says they could be persuaded to switch their vote. Their firm stance is understandable, as each group falls cleanly in line with their party's stances on many major issues. Ninety-three percent of urban liberal Democrats say gay marriage should be legal, and 85 percent prefer a "larger federal government with many services." Tea party movement Republicans are close to unanimous in their support for a "smaller federal government," and they widely oppose raising taxes on people with incomes over $250,000.

Democrats overall are less likely than Republicans to say there's a good chance they'll change their mind in 2012, but two segments of their coalition are more persuadable than others. More than 10 percent of "DIY" and  "agnostic left" Democrats say they are undecided or that they're apt to switch their current vote choice. DIY Democrats actually line up closely with Republicans on a number of issues, opposing gay marriage and preferring a smaller federal government. The agnostic left toes the party line on supporting gay marriage, but also believes in economic individualism, with 79 percent agreeing people who don't get ahead have only themselves to blame.

Some persuadable Americans -- especially "detached" independents -- are poised to have little impact on the election. While almost one in three say they could change their mind by Election Day, they're unlikely to cast a ballot on Nov. 6. Over 70 percent of this group is  not registered to vote and few plan to do so by Election Day.

The wide range in voting certainty highlights both an opportunity and a challenge for the parties heading into their national conventions: Some of the most dissatisfied independents are still trying to make up their mind about whom they will vote for, but big pockets of partisans who disagree with their party on the issues need to be brought back into the tent.

Add to that the need to energize each party's hardcore supporters who have little chance of crossing the aisle, and both conventions have a lot of work ahead of them.