Republican senators are entrenching themselves in small states that elected Democrats a few years ago, brightening the GOP's future if Americans continue their trend of voting for the same party in Senate and presidential races.
The Senate's makeup has always given disproportionate power to less populous states. As liberal voters keep migrating to urban areas, many rural states are becoming more consistently conservative, a potential problem for Democrats.
Ten years ago, for instance, all four senators from the Dakotas were Democrats. This year, Democrats are fighting an uphill battle to keep the number from falling to one.
Similarly in Arkansas, a two-term Democratic senator got clobbered in 2010. The state's last remaining congressional Democrat— Sen. Mark Pryor— is in an intense struggle for re-election.
In West Virginia, Democrats privately hold out little hope of keeping the seat that retiring Sen. Jay Rockefeller held for 30 years. In Montana they will be hard-pressed to keep the Senate seat Max Baucus had for 35 years.
All these states vote strongly Republican in presidential races. Three of them have begun bringing their Senate voting more closely in line, and Republicans hope Montana and West Virginia will follow suit.
"The small-state bias of the Senate definitely gives Republicans the advantage," said Emory University political scientist Alan Abramowitz. "Most of the least-populous states are solidly or leaning Republican," he said, while "the biggest states mostly lean Democratic."
This trend has its greatest impact in the Senate, which ignores the "one person, one vote" concept that largely drives House and presidential elections. Every state gets two senators. That means Wyoming residents have 65 times more Senate representation than do Californians.
Theoretically, a majority of senators could control the chamber while representing 18 percent of Americans. That won't happen. But there are plenty of examples of majority sentiment being thwarted, and there may be more in the future.
President Barack Obama's 2013 bid to tighten background checks on gun buyers was supported by senators representing 62 percent of the U.S. population. They included both senators from California, New York and Illinois. But partly because of smaller states' conservative leanings — and partly because of filibuster rules that protect the minority even more — 46 senators representing 38 percent of Americans killed the measure.
The Obama-backed "Dream Act" — which would have granted legal status to millions of immigrants who entered the country illegally as children — met a similar fate in 2010. Fifty-six senators representing 61.6 of the nation's population voted for it, but it failed.
The right to filibuster — which requires 60 votes to pass many measures in the 100-member Senate — killed these measures. But it does not explain the huge disparity in population represented by senators on the winning and losing sides.
It's entirely possible for senators representing more than half of all Americans to lose simple-majority votes. Last year, only 40 senators backed California Sen. Dianne Feinstein's bid to regulate assault weapons. But they represented nearly 52 percent of the population.
Of course, some low-population states lean Democratic. They, too, are tending to align their Senate votes with their presidential votes, counteracting the GOP progress.
John Chafee was a Republican senator from Rhode Island from 1976 to 1999. Republicans, or a Republican-turned-independent, held one of Vermont's Senate seats from 1857 through 2006. For 26 years ending in 1995, Oregon was represented by two moderate-to-liberal Republican senators, Mark Hatfield and Bob Packwood.
Similar scenarios are unlikely today. Those states now vote solidly Democratic in Senate and presidential races, and such moderate Republicans have harder times winning GOP primaries.
More often than not, small-population states lean conservative, especially in the West. And most big states lean Democratic, with Texas being the glaring exception.
Obama became president by carrying 13 of the 15 most populous states in 2008, and 11 in his re-election. He lost 20 of the remaining 35 states in both elections.
The Senate's "small-state advantage" dates to 1787. That's when the "Great Compromise" apportioned House seats by population, while giving each state two senators. Political parties didn't exist then.
Today, if one party can gain a decided edge in small states, it can enjoy disproportionate Senate power, even if it's outmatched in presidential elections. Republicans seem to be gaining such an edge, but it's unclear it will continue.
Gary C. Jacobson tracks these trends as a political scientist at the University of California, San Diego. In the last election, he said, the alignment of state-level voting for president and for senator was the highest since the 1950s.
In other words, states that vote Republican for president are increasingly likely to send two Republicans to the Senate. The mirror image is true for Democrats.
Looking at this year's Senate races, Jacobson said, Democrats' "only hope is that this trend isn't continuing," because they are defending seats in seven states Obama lost in 2012. Of those seven states, only North Carolina ranks among the nation's 20 most populous.
Republicans need to gain six Senate seats to control the chamber.
One encouraging sign for Republicans is the intensity with which some small states are turning to the GOP. Republican John Thune of South Dakota narrowly defeated Sen. Tom Daschle — then the Senate Democratic leader — in 2004. When Thune sought re-election six years later, he ran unopposed.
West Virginia voted Democratic for president as recently as 1996. In 2012, Obama lost the state by 26 percentage points.
If states became totally consistent in partisan voting behavior — that is, if every state either voted Republican for president and both its senators, or voted Democratic for president and both senators — Republicans would have to do a little better in presidential elections to control the Senate. Obama won 28 states in 2008, and 26 in his re-election.
University of Maryland political scientist Frances Lee said liberal voters practice "natural gerrymandering" by concentrating in urban areas.
That leaves vast stretches of America dominated by conservatives. They spread themselves among many House districts and many small and medium-density states, giving Republicans the "small-state advantage."
"The Senate is tilted against the Democratic Party in its makeup," Lee said, but Democrats have the majority because they've "overperformed," with ample help from Republican mistakes in 2010 and 2012.
Republicans say they won't make those mistakes this year.