Quite frankly, I fail to see how this would win him any support from those who don't support him presently. The pro-life crowd would love the priest and applaud the action as heroic, those who don't know about Kerry's position would be made aware of it and there could be some who don't even know that he's Catholic.
From The American Spectator:
"The Kerry campaign was said to be surprised at the coverage their candidate received for attending Mass while on vacation in Idaho. 'You saw conservatives all up in arms that he was receiving communion, when most American Catholics do the same thing and live a life very similar to the senator's: divorced, pro-choice, etcetera,' says the Kerry adviser. 'It just highlights how out of touch the right wing is with America, and we can play to that.'
To that end, according to other sources inside the Kerry camp, aides are attempting to identify a Catholic diocese, and perhaps even a specific priest and church, where Kerry could attend a Mass with reporters present, and be turned away at the altar attempting to receive communion. "