Evidence of a crime is typically the lynchpin of most criminal prosecutions. The more evidence, the stronger the case. And with some crimes the greater the evidence, the more severe the penalties. With drug prosecutions, the penalties for a few ounces of marijuana are much lower than if you are stopped with a van full of bales of marijuana.
However, sometimes the issue has less to do with the quantity of evidence and more with the source. In a case involving campaign aides to Ron Paul, prosecutors have issued new indictments on charges involving the payment of $73,000 to an Iowa senator, which was made allegedly to induce the senator to change his endorsement to Paul in 2012.
The prior case had been based on the same charges but was dismissed when a judge ruled the evidence gathered from FBI interviews was inadmissible. Such a ruling often can be the end of a case and all possible prosecution as happens when law enforcement seizes drugs without a warrant or engages in some other type of illegal search.
The exclusionary rule can be used to suppress that evidence as "fruit of the poisonous tree." This doctrine prevents the use of all evidence discovered by the illegal search or seizure. However, if law enforcement is able to discover the same evidence from a source unrelated to the illegal action, they may be permitted to introduce it from that source.
Apparently, in the case prosecuting Paul's aides, they were able to uncover emails and other communications not implicated in the dismissed interviews. When prosecutors are determined to prosecute defendants, they sometimes will be able to revive an otherwise dismissed case.
Source: nytimes.com, "Iowa: Aides to Ron Paul Campaign Charged Again," The Associated Press, November 20, 2015