Digital ads are playing an increasing role in the run for the presidency, but as the numbers show, President Obama and Mitt Romney's approaches are as different as their political positions. Without the need to wage a primary fight, Obama has blanketed the Web with ads, while his chief rival has emphasized TV and radio spots. Meantime, attention has focused on the growing role of political sites in the news ecosystem. Do they, as some charge, feed into the echo chamber of political discourse? It seems so: An examination of their audiences shows that readers tend to spend the most time on sites that reinforce the beliefs they already hold.