I am thinking that AFTER a primary if a candidate pulls out, it is the same as forfeiting the election. At one time, a party could not replace a nominee who quit for personal reasons. I am trying to think of a case where that happened.
In Louisiana in 1968, the Republicans nominated a district attorney from the Baton Rouge area to challenge Sen. Russell Long. The D.A. pulled out of the race, and the Republicans did not offer a replacement candidate. So Long was unopposed for reelection. Now, I don't think the Republicans could have offered another candidate had they tried, which they did not do.
There have been plenty of times when the primary winner dies before the general election, and nobody thinks much about it when a replacement is named.
This is only slightly different in that DeLay made himself ineligible for election. I guess he could just shoot himself. That would make the DUmmies happy.