There are two major factors that impact which way an election leans, who is in the White House and whether it's a Presidential year or a mid-term.
In a Presidential year, the party that wins almost always increases their share from the previous mid-term. This didn't happen three times, 1976, 1992, and 2000. There are reasons for these exceptions. Democrats in 1976 got the second highest share of the two party vote that any part has gotten since World War II. The highest share was in 1974. So this decline wasn't a big deal. Democrats did worse in 1992 when Bill Clinton won the White House. There are two possible explanations for this. 1) Clinton won the White House with only 43.0% of the vote. 2) The electorate was re-aligning dramatically and would lead to a Republican landslide in 1994. The last time this happened was in 2000. George W. Bush won the Presidency with only 47.9% of the vote, less than Al Gore. So it's understandable that the Republican vote share dropped.
Mid-terms almost always favor the out party. The party in the White House has only increased their vote share from the previous election once since World War II.
The way the electorate has been voting a Presidential year favors Democrats by 2-3 points compared to a mid-term electorate.
Here's who the next three elections should favor:
2014 - Electorate favors Republicans. With a Democrat in the White House, environment favors Republicans.
2016 - Electorate favors Democrats. If a Democrat wins the White House, environment will favor Democrats. If a Republican wins, environment will favor Republicans.
2018 - Electorate favors Republicans. If a Democrat wins the White House in 2016, environment will favor Republicans. If a Republican wins, environment will favor Democrats.
So the 2014 election should be the best for Republicans, while the 2016 election should be the best for Democrats. If a Republican is going to retire, the 2014 election will definitely be the best time to do it. While Jon Runyan's retirement, makes that seat more competitive now, it's far better for Republicans to compete in an open seat in NJ-3 in 2014 than in 2016.