Source: Crooks and Liars: Opinion- Dave Johnson- Democrats Who Move Right Lose Elections: There is No Center
The former great, RIP, Progressive Democratic Speaker of the House of Representatives, Tip O'Neal, had a saying, "All politics are local." Well, of course, that is not all true but a lot of it is, especially when you are talking about Congressional elections where only voters in a district or state vote for the next Representative or Senator for that seat in the next Congress.
This means that, whether you are a candidate or the incumbent, you have to know the district or state that you represent, or want to represent, in order to get elected and reelected. To put it simply, you can't be a Bernie Sanders Socialist running statewide in Mississippi and expected to get elected. Keep in mind that Senator Bernie Sanders represents Vermont, probably the most socialist leaning state in the union, with a population of around eight-hundred-thousand and only one U.S. Representative.
You can't be a religious-right candidate, who wants to incorporate their religious views into law, and expect to be elected statewide in New Hampshire, which just happens to be the most liberal-libertarian state in the Union. It, again, is small state with little more than a million people but it is the "Live Free or Die state" that doesn't like big government, economic or personal, and wants government to butt out of individual's lives.
I still give Senate Democrats the edge in retaining the U.S. Senate in November and I don't see them losing a lot of seats in the U.S. House either. I say that because, since 2005-06, they've figured out that the way to win House and Senate seats is by having their candidates represent the districts and states they are running in politically and ideologically. That means not recruiting Democrats who are as far to the left as, lets say, Elizabeth Warren, running in Red States but, in such situations, have fiscally conservative, socially moderate to liberal candidates instead.
They also recruit New Democrats, the real Liberals of the Democratic Party, the JFK/WJC wing of the party, to run in the Northeast and Midwest and even the West Coast and Mid Atlantic. Democrats who are very liberal on social issues and believe in economic opportunity and empowerment instead of government dependence. This makes them pretty liberal on economic issues as well. You recruit for the district and state instead of saying that Socialist Bernie Sanders and New Deal Progressive Elizabeth Warren can get elected anywhere in the country. That is beyond political fantasy.
The social democrat, Occupy Wall Street, wing of the Democratic Party believes in recruiting the most left Democrat possible and getting out the vote for that person. The Democratic Party has many leftist members of Congress who work only to advance their agenda and will never work with Republicans on anything. They wonder why Democrats lost 5-6 presidential elections from 1968-88 and lost the U.S. Senate in 1980 and Congress as a whole in 1994.
Senators Mark Pryor, Mary Landrieu, Kay Hagan, Mark Begich, Senate candidates Allison Grimes, Michelle Nunn, Arkansas, Louisiana, North Carolina, Alaska, Kentucky and Georgia respectfully are the potential for Democrats to hold the Senate in 2014. They are Senators from deeply red states but are perfectly tailored to represent them because these states tend to like candidates and incumbents who are somewhat independent and aren't afraid to go against their party establishment and aren't very far to the right or left politically. They all have a non-elitist, non-Washington, common person approach to politics.
Democrats hold the Senate now with a 55-45 majority because they have a lot of senators who represent red states. They could easily pick up 3-4 more with Kentucky, Georgia, Mississippi and Kansas, states with unpopular Republican incumbents because they recruit for the state instead of the party. The Republican Party hasn't figured this out yet. They have 45 senators and are in the minority because they are fanatically bound to nominate the furthest right candidate no matter the district or state.