Given the news that Rep. Foley has unexpectedly resigned from Congress and withdrawn from his race for reelection, the question of whether he can be replaced as a candidate is one governed by state law, in this case, Florida. As others have pointed out the issue is governed by Section 100.111(4)(a) of the Florida Elections Code (on page 31 of this pdf).
In a nutshell, a party committee gets to name a replacement for Representative Foley, but Foley’s name still appears on the ballot. Votes for Foley are deemed votes for his replacement. Given Foley’s association with scandal, this surely will work against Republicans: voters who are more likely to have heard about Foley’s scandal (and would vote against him) than have heard about the arcane Florida election law allowing votes cast for Foley to be counted for his Republican replacement.
A reader wrote to ask whether this somehow violates the Constitution—to have a rule that says a vote cast for X is really a vote cast for Y. I have not seen a case like that before, but my first instinct is that because this is a congressional election, the state Legislature has rather broad power to set the rules for the election (at least where Congress has not spoken). I would find it very difficult to believe that a court would set aside these rules, requiring that the replacement candidate be listed on the ballot as a matter of constitutional law. But it would be a great irony to see the Florida Republican Party go to court and ask a court to set aside the legislative rule for a federal election to protect the fundamental rights of Florida voters. If you don’t see the irony, go back and read this.