The New York Times ran a piece reporting that more Democrats running for election this year are openly campaigning on the Affordable Care Act. The piece noted that eight million people had signed up for the exchanges by the end of the open enrollment period. While this is a large base of people who may perceive themselves as benefiting from the law, it is worth noting that this number is likely to increase substantially in the months leading up to the election.
Under the law, people who face a "life event" become eligible for insurance in the exchange. Life events include job loss, divorce, death in the family, and the birth of a new child. Every month roughly four million people leave their jobs. If just one in five of these people go from a job with insurance to either being unemployed or a job without insurance, it would mean another 800,000 people are becoming eligible for the exchanges every month for this reason alone.
This means that the number of people who will have had the opportunity to buy insurance through the exchanges by election will be far higher than the number currently enrolled. Since many of these people will have found themselves unexpectedly without insurance, they are likely to especially value the opportunity to buy insurance on the exchanges.