Yes and no. It can hurt or help-depending on the Congressman.
Some Congress members want to nominate people that will be accepted by WP. If the Congressman knows about the LOA some will be more likely to nominate that candidate.
I have heard other Congressmen NOT nominate people with LOA's because they feel that that candidate is so strong that WP will get them a nomination from another source and hence do not want to "waste" their nomination on a person who will get in WP by other means.
This is dangerous "gamesmanship" by Congressmen but some do it in hopes of getting two people from their District in WP.