In 2000 Al Gore won 18 states plus the District of Columbia and lost the electoral college by 5 votes, (assuming that you accept the totals from Floria). In 2004 John Kerry won 17 states plus the District of Columbia and lost the electoral college by 25 votes. The question that is facing Democrats isn't which candidate can get the most votes, its which candidate can win the electoral college.
So far, there is a case to be made by each candidate. The one made by Obama is that he is showing strength in states that Democrats don't normally win, such as Kansas, and if he could duplicate that effort in the fall, he would put many more states in play and thus increase his chances of winning the presidency. The counter-argument is that a lot of his wins in so called "red" states have been in caucuses and such results are not a true indicator of how he will run in the fall.
The case for Clinton is that she is showing strength in states that Democrats absolutely have to carry, and she is showing strength with Latino voters. The latter could become very important because she could take Florida, New Mexico, and Colorado, which would give her the presidency. The counter-argument is that she is showing strength in states that Obama can also carry if he is the Democratic nominee and so is adding nothing to the electoral mix.
Ohio is a very important state in the GOP's calculations. No Republican has ever won the presidency without carrying Ohio. It has a very diverse population and if Obama or Clinton could win Ohio, then he or she would probably also win Missouri and maybe Florida. Those three states would guarantee the presidency for the Dems.
So while electability is important, it isn't popular vote electability, it is who can take enough states to win the electoral college. Ohio Dems could do a lot worse than trying to read polls of Ohio voters showing the results in match-ups between McCain and Clinton and McCain and Obama.