Most of the Democratic fretting thus far in the election season -- and with nine months to go, it's still early on -- has been focused on potentially losing their 39-seat majority in the House. After all, Democrats control the Senate, where incumbency rules, by a comfortable 10 votes, and only 37 seats are up for grabs this year. However, recent retirements, worrisome poll numbers for several incumbents and the entrance of some strong GOP challengers have prompted a wave of concern for Senate Dems. While still unlikely, if a perfect political storm were to occur, there is a chance that Democrats could lose the Senate. Here's what would need to happen:

Open seats likely to switch: At least two seats are all but write-offs for Democrats: North Dakota and Delaware. Delaware Attorney General Beau Biden's decision not to run for his father's old job means Dems have lost their best hope at keeping the seat from the popular, eight-term moderate Republican Congressman Mike Castle. And Byron Dorgan's retirement opens up a blue seat in a red state in a year that's trending red. Dems had no better candidate than the three-term Dorgan, who felt that even he couldn't win in this climate. "North Dakota is pretty much gone, and Delaware has become very difficult," says Jennifer Duffy, Senate editor at the nonpartisan Cook Political Report, which tracks congressional races. "This gets Republicans to 43."

Vulnerable swing-state Democratic incumbents: Senate majority leader Harry Reid leads the list of vulnerable incumbents. The Nevada Democrat trails by double digits in theoretical matchups with not one but two would-be GOP challengers -- a former Miss America runner-up and the son of a legendary but scandal-plagued college basketball coach, which gives you an idea of how shallow the Nevada GOP bench is and how much trouble Reid is in. Despite the millions of dollars he has already spent on TV and radio ads promoting all he's done for the Silver State, his approval ratings have hovered near the 30% mark. As a mark of how worried Dems are, when asked repeatedly to comment about potential Democratic losses, Eric Schultz, a spokesman for the Democratic Senatorial Campaign Committee, didn't return the e-mails.