By now, it's accepted wisdom that Democrats perform better in presidential elections, when the electorate is more diverse and younger, while the GOP's strength is in midterm elections, when their core voters are likelier to turn out. But it's worth remembering that this is a recent phenomenon, thanks to the changing makeup of the Democratic Party under Barack Obama's presidency — and there's no guarantee it will continue unabated.
It wasn't the case in 2004, when George W. Bush effectively mobilized conservative voters to overcome growing public dissatisfaction toward his presidency. It wasn't the case in 2006, when the Democratic Party capitalized on increased support from older, white voters to retake the House and Senate. And it wasn't the case in the decades prior, when Democrats often recorded significant gains or outperformed expectations in midterm years (1982, 1986, 1998), while Republicans won five of seven presidential elections from 1980 to 2004.
What's changed is the makeup of both parties' coalitions. Seniors, who frequently voted Democratic over pocketbook issues like Social Security and Medicare, have migrated into the Republican column. White blue-collar voters, once a staple of Democratic coalitions past, have become estranged from their old political home over cultural issues. In their place are what my colleague Ron Brownstein labels "the coalition of the ascendant" — single women, minorities, and millennial voters. Voters within these groups turned out at high levels in the last two presidential elections to offset Democratic losses elsewhere.