The chart below shows the number of seats that the two major parties are likely to win if the 2014 election favors Democrats by 54% to 46% (like 2008), if it is an even partisan year, and if it is a year that favors Republicans by 54% to 46% (like 2010). Note the difference in effect between a 4% shift toward each of the two parties.
For more information on each individual congressional district and FairVote's methodology in producing these projections, see its Monopoly Politics 2014 spreadsheet - embedded online and also available fordownload. The spreadsheet allows users to manually change the overall partisan lean of the 2014 election, as shown in the chart above. It also gives users the capability to change the average advantage that incumbents will receive over challengers (the "incumbency bump"), reflecting the overall attitude of voters toward incumbents.