Stuart Rothenberg, who has been a political analyst for CNN and CBS News, is editor and publisher of The Rothenberg Political Report, a nonpartisan political newsletter.
WASHINGTON (CNN) -- Every election cycle, a handful of races not taken very seriously early on become truly competitive contests, and this year is no exception.
Every candidate, of course, has a scenario. But most of those scenarios -- and most of those candidates -- evaporate well before October.
In the case of the candidates below, their scenarios are now supported by both polling and successful fundraising. Given the national political environment, it isn't surprising that most of the late surprises are in Republican-held seats.
It's likely that not all of the House races below will change party hands on November 4. In fact, at the end of the day, none of these potential surprises is guaranteed to be a winner.
Still, as Election Day approaches, they are worth watching if only because they are a way of measuring exactly how well the two parties are doing.
Idaho's 1st Congressional District: Walter Minnick's resume of accomplishment and substantial personal financial resources weren't expected to be enough to make this into a competitive race. While he holds a law degree from Harvard, is a successful businessman and is a former deputy assistant director of the Office of Management and Budget, Minnick is running as a Democrat in a district that gave President Bush 68 percent of its vote four years ago.
But polls show Minnick running about even or ahead of incumbent Republican Rep. Bill Sali, giving Democrats a real chance for an upset.
The national mood for change certainly is helping Minnick, but even more important is Sali, a conservative who barely won two years ago. He has alienated members of his own party with his no compromise approach, and he'll have to lose thousands of strong Republican voters to lose his seat. It's quite possible that will happen.
South Carolina's 1st District: Republican Henry Brown served for 15 years in the state Legislature before being elected to Congress in 2000. His district, which includes Charleston, Myrtle Beach and other coastal areas, has been a Republican area for years. But it has been attracting many affluent folks from out of state who have shown more political independence.
Democrat Linda Ketner, a businesswoman and openly gay activist for women's issues and social justice causes, raised more than $1.5 million through the end of September and loaned her campaign $850,000 so far, making this a serious contest. She has scored strongly by attacking Brown for starting a fire during a windy day that spread to the Francis Marion National Forest, and for refusing to accept responsibility.
Minnesota's 6th District: Even Democratic strategists had just about written off former Blaine Mayor Elwyn Tinklenberg's challenge to Rep. Michele Bachmann. Bachmann, who served in the state Senate before being elected to Congress two years ago, is an outspoken conservative in a district that is a Republican stronghold.
But when Bachmann recently said on cable television that Sen. Barack Obama and others in Congress are anti-American, she unleashed a fury of opposition. Democratic fundraising surged and polling quickly showed the race turning around. The congresswoman has tried to backtrack, but the damage was done. Now, instead of coasting to victory, Bachmann is fighting for her life. She may not survive politically. Watch as Bachmann courts controversy with her comments on Obama »
Arizona's 3rd District: When Republicans selected Sen. John McCain to be their party's nominee for president, most observers figured that Arizona Democratic incumbents and challengers would be in deep trouble. But attorney Bob Lord refused to buy that analysis, and his determination has paid off.
McCain's poll numbers have weakened dramatically, and he's providing absolutely no lift to Republicans in his home state.
Lord now looks to be a serious threat to incumbent John Shadegg, an outspoken conservative who was first elected to Congress in 1994 and who ran unsuccessfully for House Republican leader after Tom DeLay gave up that post.