In Florida, Democrats found themselves suddenly competitive in a district where Foley, 52, had been considered a shoo-in.
His resignation further complicates the political landscape for Republicans, who are fighting to retain control of Congress. Democrats need to win a net of 15 Republican seats to regain the power they lost in 1994.
Foley's West Palm Beach district is largely Republican, but now may be won by a Democrat.
State Republican leaders chose a state lawmaker Monday as a replacement for Foley, whose name remains on the November ballot for the seat he vacated last week after reports that he sent sexual messages to male teenage pages.
State Rep. Joe Negron of Stuart will get any votes that go to Foley, who said he sought treatment for alcoholism and accepted responsibility for his actions.
Tim Mahoney, a former Republican and financial services adviser, is the Democratic nominee for the 16th Congressional District seat held for the past dozen years by Foley, who was regarded a shoo-in for re-election before his resignation Friday.
"Obviously I'd rather have my name on the ballot, but Mark Foley, that name is a placeholder — that's it," Negron said in Orlando before a decision was announced. "He's withdrawn — not in Congress, not a congressman any more. This is going to be a race between Joe Negron and Tim Mahoney."
Negron also said that Mahoney helped the GOP by campaigning Saturday with John Kerry and liberal U.S. Rep. Debbie Wasserman Schultz, the Palm Beach Post reported.
"I think (Mahoney) is telling the voters of District 16 what part of the Democratic Party he comes from," said Negron.
Mahoney didn't return several phone calls seeking comment.
Some Republicans think it might be wiser to forfeit Foley's seat and avoid keeping the story alive through Election Day.
"I think it's a death sentence ... mission impossible," former state Republican chairman Tom Slade said. "The only way you win is they (voters) have got to vote for Mark Foley. That doesn't appear to me to be very attractive."
But state party spokesman Jeff Sadosky said the seat is too important to surrender.
"The voters will educate themselves running in the race, not the candidates from the race in the past," Sadosky said. "It's not going to be about yesterday's news no matter how tragic and horrifying."
When asked whether the Foley scandal contributes to a "perfect storm" brewing for Republicans before the midterm elections, White House press secretary Tony Snow was dismissive of the idea.
"Mark Foley has got to answer for his behavior, right? Now this does not affect every Republican in the United States of America, just as bad behavior on the parts of Democrats in ages past has not been a reflection of their entire party. These are things that happen, and they need to be addressed and the individuals responsible," he said.