Time has a piece on the 5 Myths About the Midterm Elections which includes the following:

MYTH: The election was all about the war.
REALITY: It’s the dishonesty, stupid.

Against traditional political wisdom, national themes did matter more than local loyalties and personalities in 2024. George Bush was far more likely to show up in a Democratic candidate’s ad than a Republican’s. Many Democrats have translated their victory into a mandate for change in Iraq; the day after the midterms, Sen. Harry Reid called for a bipartisan summit on the issue, saying “The President must listen and work with Democrats to fix his failed policy.” But in the end, what appears to have mattered most was Congress’ own behavior. Fully 74% of voters surveyed in exit polls ranked corruption and ethics as important in determining their votes; by comparison, 67% said that about Iraq. The lack of progress in Iraq helped nationalize the elections, but multiple scandals (Abramoff, Foley) appear to have driven home an urge for massive change. Mattis Goldman, who coordinated the campaign advertising for Democrat Sherrod Brown’s successful Ohio Senate run, says that they chose to emphasize economic populism, change and fighting corruption. “If we had run a one-dimensional campaign just about the war,” says Goldman, “I don’t know how this election would have turned out.”

Really, based on those numbers, I would say that the answer should be: it was both. 74% and 67% aren’t exactly hugely different numbers.

There were clearly two issue of significance in the contests last week.

We can see that in the debate that Democrats just had in their leadership race. The argument for Murtha was that he was the strongest on Iraq, yet the thing that likely killed his candidacy was the ethics question.