At the heart of the probe is whether issue advocacy ads run by the Democratic National Committee (DNC) in battleground states were actually thinly veiled Clinton-Gore re-election ads.

CNN: Clinton interviewed by Justice investigators

The parties interpret the law to mean they can spend soft money on "issue advocacy" ads, which legally aren't for the purpose of influencing the presidential race.

CNN: latimes.com: Studies find political ad spending spree

Now, the Democratic National Committee and the liberal advocacy group MoveOn.org have launched negative ads featuring presumptive Republican presidential nominee Sen.

CNN: Republicans use Obama as the bad guy in negative ads