In an interview at the Capitol’s Speaker’s Lobby, just off the floor of the House of Representatives, Rep. Mario Díaz-Balart, a Miami Republican, said he was skeptical about Nieves’ request for asylum considering that his role “in the oppression machine of going after political opponents” was very recent. He noted that López was sentenced to nearly 14 years in prison only two months ago.
“We have to be very, very careful to make sure that anybody who seeks asylum in the United States is in fact somebody who deserves asylum, not someone who is part of the regime,” Díaz-Balart said.
The point I make is that from a strategic perspective you deal with dirty people in order to get dirt. If Nieves has concrete evidence (documents, for example) then I would be surprised if he was rejected by those who want to discredit the Maduro government. This is especially true because of the timing--a bombshell just a few weeks before the legislative elections would be highly valuable to the opposition, even if they despised the messenger.
To be more specific, if Nieves can prove that Leopoldo López was put in jail simply because Maduro wanted him there, he gives a boost to the opposition because of Ni Nis, those Venezuelan voters who are skeptical of both the government and the opposition. Even if it doesn't prompt them to vote for opposition candidates, disgust may keep them at home rather than voting for government candidates.
Now, if he doesn't have concrete evidence and he's just a disgruntled person (the Venezuelan government claims he was bribed) then I don't know how much impact it will have. Likely more than zero, though, which may well still make it worth the opposition's while.