(CNN) - Former President Bill Clinton will join President Barack Obama at next Wednesday's Senate Democratic retreat at the Washington Nationals stadium, according to two Democratic sources.
Most of the discussion at the retreat will revolve around messaging for the Democrats' 2014 middle class agenda, a Democratic aide told CNN.
The twin presidential appearances come at a time of heightened anxiety for Democrats analyzing the midterm elections and their party's ability to preserve its slender Senate majority.
The Democratic caucus currently holds a 55-45 majority in the 100-member chamber, but 21 Democratic-controlled seats are up this year as opposed to only 15 Republican seats. Making matters worse for Democrats is the consensus view that the overwhelming majority of vulnerable seats are currently held by Democrats.
Arkansas's Mark Pryor, Louisiana's Mary Landrieu, Alaska's Mark Begich and North Carolina's Kay Hagan are widely seen as the most vulnerable incumbents seeking re-election this year. Democrats also face an extremely stiff challenge holding on to open seats in South Dakota, Montana and West Virginia.
Republicans are also hoping to make a serious run at Democratic seats in Iowa, New Hampshire, Virginia, and Michigan.
The most vulnerable Republican seats up this year are those held by Senate Minority Leader Mitch McConnell of Kentucky and Saxby Chambliss of Georgia, who is retiring.
Senate Republicans will also hold a one-day "issues conference" on Wednesday at the Library of Congress.
CNN's Jim Acosta, Ted Barrett and Alan Silverleib contributed to this report.