Much of the behavior of Democratic Senate candidates can be explained by one simple fact: very few of them are going to get 50% of the vote this fall. Even the candidates who have a good chance to win are going to struggle to get to 50%. Let's take a quick look at the RealClearPolitics polling averages for the 22 Senate races that RCP lists as being in play, or where there's no average the last listed (generally Rasmussen) poll, just focusing on the Democratic candidate - as you will see, in only 4 of those 22 races is the Democrat polling 50/50 or better, and in only 7 of the 22 is the Democrat even polling above 43%:
These are the few popular Democrats. Wyden, Blumenthal and Manchin are all popular figures (an incumbent Senator, AG and Governor, respectively). The first two are solid favorites; Manchin could face a tough race if the GOP can tie him to the Obama Administration, which is deeply unpopular in West Virginia, but for now he's in a good place. Gillibrand has suffered soft approval ratings but for now has little known opposition.
Three incumbents in jeopardy (an incumbent polling below 50 is always considered in danger) and facing vigorous challenges, but all stand a fighting chance. Harry Reid, while he's revived from being down below 40%, simply can't crack 50% - neither can his son, running for governor - which is why his strategy is almost entirely built around the national media battering Sharron Angle and letting third-party candidates siphon off enough votes to let him win below 50 as he did in 1998.
Only Feingold and Bennet are incumbents here, and so a number of these races still have a lot of undecideds. (In Colorado, the Senate primary is something of a proxy war, with President Obama actively backing Bennet after failing to bribe Romanoff to drop out of the race, while former President Clinton is backing Romanoff). But again, voters don't seem too enamored of any of them, which is why the Democrats will be running an almost entirely negative fall campaign focused on driving small numbers of voters from the GOP to third parties.
FL-Crist 37/36%, Meek 16%, Greene 17%
Not all of these races are uncompetitive - incumbent Richard Burr is listed at a weak 46% in North Carolina, Mark Kirk is even below Giannoulias in Illinois, and of course in Florida the establishment Democrats are abandoning their own candidates to line up behind the incumbent, nominally Republican governor (a man the DNC stood ready to demonize if McCain had chosen him as a running mate in 2008), Charlie Crist. But once again, the divide-and-conquer strategy is basically the only way they can play this hand.
If the Democrats manage to hold onto the Senate, it won't be because the people of more than a tiny number of the states voting this November have actually given their performance a thumbs-up.