Washington ( CNN ) -- It was supposed to be a cakewalk for Eric Cantor . The analysts said so . The polls said so . Cantor felt so comfortable that the House majority leader was n't in his Richmond , Virginia , district in June on the day of his Republican primary election . He lost -- badly -- to a little-known economics professor in one of the great American political upsets . How did the pundits and pollsters get it so wrong , and what does that mean for November 's congressional elections ? Here are five reasons to approach any prognostication with caution this time around : . 1 ) Low turnout . While more than half of eligible voters turn out for presidential elections , the percentage drops sharply in midterm balloting for all 435 House seats and about a third of the Senate . In Cantor 's 7th District race in Virginia , well below 20 % of eligible voters cast ballots on the day he lost . Such a low turnout makes the outcome more vulnerable to unpredictable factors such as bad weather , a campaign misstep or crossover voting -- in this case , Democrats who may have voted for tea party-backed David Brat in an effort to oust Cantor , the seven-term congressman considered a possible future House Speaker . Brat beat Cantor by more than 7,000 votes out of 65,000 cast . In 2012 , the turnout for the district 's Republican primary was about 47,000 votes , and there was no way to determine how many of the 18,000 additional voters this year were Democrats . 2 ) National mood . Most observers think the more public distaste for Congress and politics , the lower the turnout in congressional elections . By that measure , polling stations may be particularly barren come November 4 . Approval for Congress is near record lows , and a Washington Post-ABC poll in early August showed that for the first time , a majority of Americans disapproved of their own representatives . `` Americans like to vote for someone , not against someone , '' said CNN Polling Director Keating Holland . `` If they perceive the ballot as a choice between the lesser of two evils , that 's one more excuse not to vote , and in midterms a lot of Americans do n't need many excuses to simply stay at home . '' Facing such a disgruntled electorate , both parties seek to energize their respective bases to try to gain a turnout advantage . Republicans frame the election as the chance to increase their House majority and regain control of the Senate , which would strengthen the party 's ability to minimize or defeat the policies of Democratic President Barack Obama . Democrats reach out to demographics that traditionally support them -- single women , minorities and younger voters -- but traditionally do n't turn out strongly in midterm elections . 3 ) Polling problems . Changing dynamics make effective polling more challenging . Cantor 's pollster badly underestimated the turnout for June 's primary , perhaps because he failed to anticipate Democratic crossovers and conservative disenchantment with his candidate . Whatever the reason , difficulty in knowing who will vote makes predicting the outcome a major challenge . Along with unorthodox tactics such as crossover voting , pollsters have a tougher time reaching a growing number of prospective voters through traditional methods . For example , fewer young voters have land-line telephones , the traditional polling medium for decades , meaning surveys must adapt by incorporating cellphone and Internet outreach to properly reflect the population . 4 ) Beware of the aggregate polls . Many popular websites simply take whatever polls they can find and average them together . Some use more sophisticated mathematical models , but the principle is the same : The end product is only as good as the data it is based on . `` Computer programmers used to say , 'Garbage in , garbage out , ' and that 's true for virtually any mathematical model , '' Holland said . `` Let 's say you have 10 polls in a state and five of them are partisan polls that are trying to spin the data for their candidate , three of them are cutting corners on their methodology , one of them is flat-out making the data up , and only one is a legitimate poll . '' The lone legitimate poll will get `` swamped '' in any aggregate model by the questionable polls , he continued , adding : `` Unfortunately , there are at least a few states where the skunky polls vastly outnumber the good ones . '' 5 ) It 's all local . `` All politics is local , '' former House Speaker Tip O'Neill famously said , and that is particularly true in congressional elections . While voting takes place in all 50 states , the competition involves the district congressional races and statewide Senate races . That means an outcome of national significance can hinge on the result from a particular district or state . For example , which party wins a majority in the Senate depends on the outcomes of half a dozen races , including contests involving incumbent Democrats in Arkansas and North Carolina . `` If it 's sunny and warm in 48 states on Election Day and cold and rainy in Arkansas and North Carolina , control of the Senate could be determined by intensely local weather conditions , '' Holland noted . CNN 's Dan Merica and Paul Steinhauser contributed to this report .
Low turnout in congressional elections can bring surprises . Public distaste for Congress could mean fewer voters in November . Comment: `` All politics is local , '' as former House Speaker Tip O'Neill said .