What makes a battleground state a battleground state? For one, obviously, presidential races decided by narrow victory margins. But another way is to examine how a state has voted in other statewide elections. Does a state tend to only elect Democrats, only Republicans, or a mixture?

The most widely circulated bit of information from this year’s installment of National Journal‘s Congressional vote ratings was that Barack Obama had the most liberal voting record in the U.S. Senate, narrowly beating out Rhode Island’s Sheldon Whitehouse and former presidential candidate Joe Biden from Delaware.

But another interesting tidbit is that most of the battleground states in this year’s presidential election are also currently represented in the U.S. Senate by both a Republican and a Democrat. Moreover, the difference in liberal / conservative voting records between these Senators within a state is fairly large; meaning, these states are electing both fairly conservative and fairly liberal Senators to D.C.

Leading the way is the battleground state of Nevada – Republican John Ensign’s voting record was deemed the 13th most conservative in the chamber, while Majority Leader Harry Reid’s was measured as the 9th most liberal. The liberal / conservative ‘gap’ between the two Senators was 73.7 points (on a scale of 100).

Second on the list was the state of Iowa – Republican Charles Grassley was the 25th most conservative Senator, while Democrat Tom Harkin (up for election in 2008) was the 11th most liberal. The liberal / conservative gap for Iowa’s Senators was 62.4 points.

Going on down the line of the states with the largest ideological difference between its U.S. Senators is a string of classic battleground states: Colorado, New Mexico, Missouri, Florida, Minnesota, Ohio, Oregon, Pennsylvania, and Virginia. Only two of the states on the list – Louisiana and Indiana – are generally considered safe Republican territory in Presidential elections – although Barack Obama is eyeing both in 2008.

While both Republicans and Democrats currently represent these dozen or so battleground states in the U.S. Senate, the trend within these states is not favorable for the GOP. Nearly half of the states on the list elected Democrats in its last Senatorial election: Colorado (Ken Salazar), Missouri (Claire McCaskill), Minnesota (Amy Klobuchar), Ohio (Sherrod Brown), Pennsylvania (Bob Casey), and Virginia (Jim Webb). Democrats are also expected to pick up open seats in Colorado, New Mexico, and Virginia this year, as well as compete strongly against incumbents in Minnesota and Oregon.