There is no key voting bloc
Yes, any candidate would do better if more of a certain demographic voted for them. But that doesn't mean they should automatically target that demographic. Maybe they'd be better off increasing turnout among demographics they're already strong in. Or targeting demographics where the voters are most persuadable. Or targeting demographics that can be appealed to with messages that don't jeopardize the candidates standing with other groups.
Yglesias says something similar here:
...[T]he accompanying analysis says "Barack Obama's appeal to younger voters and John McCain's support among older voters may have created a situation where the outcome will turn on the preferences of middle-aged voters -- particularly those in their 40s." You see analysis of this sort all the time, but it's all based on a mistake -- there's not a demographic electoral college where "winning" particular sub-samples of the population is the key to victory and therefore it's important to focus attention on the most evenly divided demographic groups. If John McCain persuades an Obama-supporting 25 year-old to switch to his camp, that has just as big an impact as one 45 year-old one 65 year-old or one 85 year-old.You hear pundits making this kind of mistake all the time. They decide the soccer moms, or NASCAR dads, or turnout of young people, or whatever is the key to everything. In a close race, they'll pick one particular subgroup and claim it's responsible for the out come. No, it's not. Everything contributes.
Beyond that, if you do want to label any particular group as key (for the sake of deciding which TV shows to advertise on, for example) the reasonable approach isn't to look for closely divided groups, it's to look for groups with lots of people who haven't stated a preference on the theory that those people might be easier to persuade. Voters over sixty have a marked predilection for John McCain, but there are also a lot of undecided voters in this bloc that might be worth going after. For either campaign, who "wins" seniors is irrelevant, you just go after persuadable voters, and it's arguably among seniors where the biggest group of persuadables is.
This comes up a lot with the 2000 election with Nader pulling votes from Gore. Certainly, Nader was a factor, but there are a zillion other things that could have compensated but didn't. A 0.5% increase in Hispanic women supporting Gore in Florida might have swung the outcome the other way. But no one blames Hispanic Floridian women, or any other demographic group, for Gore not getting enough votes to prevent the Supreme Court from intervening and awarding the state to Bush.