But as Tuesday night showed, from time to time, you really just didn't see that one coming.
Here are some other examples of shocking upsets. What did we miss (and we're sure we missed a lot)? The comments section awaits...
Regina Seltzer: The 71-year-old librarian upset three-term Rep. Michael Forbes (D-N.Y.) in a 2000 primary after Forbes switched from the Republican Party. Seltzer raised $40,000, compared to Forbes's $1.4 million.
Jesse Ventura: The third-party candidate and former professional wrestler beat Republican Norm Coleman and Democrat Skip Humphrey in the 1998 Minnesota governor's race. This despite the fact that Coleman was a future senator and Humphrey a member of the First Family of Minnesota politics.
Harry Truman: We don't really have to go over that whole newspaper headline thing, do we?
Jimmy Carter: The little-known one-term Georgia governor beat a field of better-known Democratic candidates in the 1976 presidential primary and then became one of the more unlikely presidents in history.
George Nethercutt: The first-time GOP candidate's upset win over then-House Speaker Tom Foley (D-Wash.) in 1994 was the icing on the cake for the Republican Revolution.
Harris Wofford: The little-known and recently appointed Democratic senator began his 1991 special election campaign against former governor and U.S. attorney general Dick Thornburgh (R) down by more than 40 points. He won by 10.
George Pataki: The same year as Nethercutt, Pataki pulled an arguably bigger upset in the New York governor's race. The former Peekskill mayor and state legislator beat then-governor Mario Cuomo (D), who was a titan of Democratic politics and was seen as the frontrunner for the Democratic presidential nomination in both 1988 and 1992, opting not to run both times.
Scott Brown: Speaking of state legislators pulling upsets, Brown's 2010 special Senate election win over state Attorney General Martha Coakley in deep-blue Massachusetts remains perhaps the biggest shock in recent years. All the more impressive: Brown won the seat that had been held by liberal lion Ted Kennedy for decades.
Melissa Bean: Bean lost to longtime Rep. Phil Crane (R-Ill.) by 14 points in 2002, but came back two years later to beat him by four. Crane had been in Congress for more than three decades and, at the time, was the longest-serving House Republican.
Joe Biden: Before Joe Biden was Vice President Biden, he was a 29-year-old kid upsetting incumbent Sen. Caleb Boggs (R) in 1972 using a family-run campaign with little funding. Biden trailed big as late as the summer of '72.