From al-Qaeda’s perspective, there was really no need to attack the polling places, however, as the vote itself is liable to be more destabilizing for the country than anything they could’ve done.
The 2010 election saw a Sunni-dominated secular bloc winning the popular vote, and the US imposing a power-sharing deal that left Prime Minister Nouri al-Maliki the ruler. The power-sharing never came.
This time around, many of the Sunnis didn’t even get to vote. Most of the polling stations in the Anbar Province never opened, with al-Qaeda controlling the major cities there. Resentment over seeing their election win in 2010 go for naught is likely to pale in comparison to not getting to vote at all.
Prime Minister Nouri al-Maliki not only didn’t follow through on power-sharing, but centralized power greatly in the past few years, and it is now seen as a given that he will remain, no matter what the results of the election actually are.
The post-vote coalition negotiations are going to be difficult, with no one likely to willingly deal with Maliki after the last time, and no group likely to successfully take power without his permission.