Even as House Democrats yesterday revealed their version of a major overhaul to the nation's healthcare system, activists on both sides of the debate are marshaling a potent force to push their viewpoints—senior citizens. Polls have indicated that concerns about the overhaul are strongest among senior citizens, an influential bloc because people 65 and older are the most reliable voters. Their power is magnified in midterm elections because turnout is usually much lower than in presidential years. That is why America's Health Insurance Plans is spending millions to run these advertisements in 10 states.