Nine-term Rep. Mike Castle of Delaware is the third prominent Republican to consider a third-party bid this year after a suffering a stinging setback at the hands of tea-party-backed conservatives.
If Castle decides to make an independent run for Senate, he will join Florida Gov. Charlie Crist and Alaska Sen. Lisa Murkowski in refusing to let GOP primary voters force them into retirement.
While Crist, Murkowski and Castle are well-known politicians, many third-party campaigns are lonely, low-budget affairs with little hope of winning more than 2 or 3 percent of the vote. But in extremely tight races, that could be enough to swing the outcome between the Democratic and Republican nominees.
In several cases, Democrats hope to benefit from third-party campaigns by conservatives with tea party ties, who threaten to pull votes from the Republican nominees.
That's the hope of two hard-pressed House Democrats in Virginia, Tom Perriello and Glenn Nye. Their well-financed GOP opponents failed to persuade one or more of their unsuccessful Republican rivals from mounting independent campaigns, which conceivably could siphon away enough conservative votes to sink the GOP nominee.
Third-party candidates rarely win, but they're sometimes successful "in pulling the rug out from under the nominee" who defeated them in the party primary, said Ross K. Baker, a Rutgers University political scientist. "They're disgruntled primary losers, and they want revenge," he said.
Among the nation's most famous, or infamous, third-party efforts was Ralph Nader's 2000 presidential bid. He won enough liberal votes in Florida to keep Democrat Al Gore from carrying the state and becoming president.
Connecticut voters elected third-party candidates Lowell Weicker as governor and Joe Lieberman as senator, but few other Americans have matched their success.
This year's most visible third-party campaigns are being mounted by prominent Republicans who fell victim to tea party-backed candidates who labeled them as too accommodating to Democrats.
Crist left the Republican Party in April when it became clear that Marco Rubio, a tea party favorite, would win the party's Senate nomination. Republican officials hope Crist's independent campaign will pull about as many votes from Democrat Kendrick Meek as from Rubio. That probably would lead to a Rubio win, unless Crist can take huge numbers from both rivals.
More recently, Murkowski, who lost the Alaska GOP nomination to tea party favorite Joe Miller, has launched a write-in campaign to try to keep the seat she has held since 2002. Political insiders say the effort probably will fail, as has every Senate write-in campaign since 1954.
But it's hard to predict whether Murkowski would pull more votes from Miller or from Democrat Scott McAdams, leaving Miller's front-runner status slightly in doubt.
On Friday, top GOP officials were urging Castle to drop the notion of a write-in campaign after his stunning loss to insurgent conservative Christine O'Donnell in the Senate primary.
"I just think write-ins are long shots," said Sen. John Cornyn of Texas, head of the Republican Senatorial Campaign Committee. "As chairman of a party committee, it is our responsibility to support the nominee, the choice of the primary voters, and that's what we're going to do."
Some third-party candidates are hurting Democrats as well. A Green Party candidate in Arkansas is likely to take votes from Democratic Sen. Blanche Lincoln, who is trailing Republican John Boozman in polls.
In House races, Republicans hope to benefit from a Hispanic independent candidate who might pull votes from Rep. Loretta Sanchez, D-Calif. In a Michigan race, a tea party candidate and a Green Party candidate might essentially cancel each other out in Democratic Rep. Mark Schauer's re-election battle against Republican Tim Walberg.
Third-party candidates also are affecting governors' races. In Colorado, Republicans say anti-immigrant crusader Tom Tancredo killed the party's already slim hopes in the open gubernatorial contest by running as an independent this fall.
In Massachusetts, independent Timothy Cahill vows to stay in the gubernatorial race despite the resignations of two top staffers. Some Republicans worry that he will hurt GOP challenger Charles Baker's effort to oust Gov. Deval Patrick, D.
In Minnesota, Tom Horner, a moderate former Republican, is running an impressive independent bid for the open governor's seat. He trails Democrat Mark Dayton and Republican Tom Emmer in polls, but both see Horner as a possible threat. Minnesotans elected independent Jesse Ventura as governor 12 years ago.
Texas Republican Gov. Rick Perry is favored to win re-election on Nov. 2, but supporters are keeping a wary eye on Libertarian candidate Kathie Glass. A strong showing by her might help Democrat Bill White.
Rhode Island's gubernatorial race features two prominent third-party candidates: independent Lincoln Chafee, a former GOP U.S. senator; and Moderate Party candidate Ken Block. The Republican nominee is John Robitaille, and the Democrat is Frank Caprio.
Elsewhere, long-shot third-party candidates conceivably could cause headaches for Republican Senate nominee Roy Blunt in Missouri and Sen. David Vitter, R-La.
Finally, the oddest of alternative choices could swing the intensely watched Senate race in Nevada, where Senate Majority Leader Harry Reid is fighting for his political life against Republican Sharron Angle, a tea party favorite.
Nevada voters can choose "none of the above" on their ballots. Democrats think a number of voters who dislike Reid will find Angle too extreme and inexperienced, and they will choose "none of the above" to register their discontent.
If enough voters do so, it just might hand Reid a fifth term.
Associated Press writers Becky Bohrer in Juneau, Alaska, and Jim Kuhnhenn and Liz Sidoti in Washington contributed to this report.