Summertime during presidential campaigns doesn’t have a particularly proud tradition of being constructive, but this year’s negativity nonetheless reflects some emerging political realities.

“Neither side is going to say that out loud, but the idea is to keep independent-minded people from turning out because they’re just so disgusted about the process,” said David Mark, the editor of the website Politix, who wrote a book on the purpose and effectiveness of negative campaigning. “Both sides seemed to reach a conclusion that there aren’t that many more independent voters, so they’re focused on turning out their base.”

That helps explain why Romney’s selection of Ryan has done little to shift the polls or the arc of the campaign. The pick was directed more toward exciting conservatives than it was about winning over independents.