The chart above traces the popular vote totals of Democratic and Republican presidential tickets from 1976 to 2012.
For the Democrats the general trend lines have been relatively simple. After a decline in vote totals in 1980 (Jimmy Carter's defeat), from 1984 to 2008 the respective Democratic candidates improved the vote totals from the four years prior. In 2012, four years after Obama received a record-setting 69 million plus popular votes, his successful re-election effort garnered around 3 million fewer votes. But that aside, for 7 consecutive elections, the Democrats improved their vote totals, relative to the Democratic totals in the prior election. It's significant, though, that of those 7 contests, the Democratic candidates only won 3, those being Clinton's wins in 1992 and 1996, and Obama's victory in 2008.
The Republican trends were a bit more complex. In 1980 and 1984, Ronald Reagan's victories also saw vote gains over the four years prior. But in 1988 and 1992, George H.W. Bush received fewer popular votes than the totals four years prior. In 1988, Bush won a popular vote and electoral vote landslide, but relative to Reagan's 1984 GOP-best performance, it was a popular vote decline. GOP vote totals in the defeats of 1992 and 1996 were well below the GOP 1988 totals, thus constituting the worst GOP popular vote totals of the last quarter-century.
In 2000 and 2004, GOP popular vote totals rebounded, and in 2004, garnered a GOP-best 62 million votes. In 2008 the McCain-Palin ticket received around 59 million votes, a decline from George W. Bush's 2004 totals. In 2012, the Romney-Ryan GOP ticket did outperform the 2008 GOP ticket.
If there's any simple good news for both of the two major U.S. parties, it's that each party has received its three highest popular vote totals in the last three elections. For the GOP, its best year was 2004, followed by 2012 and 2008. The Democrats had their best performance in 2008, followed by 2012 and 2004.