While Senate Democrats stand to lose a handful of seats next year, their 59-seat stronghold is threatened even before then — in the lame-duck session set to begin Nov. 15.

In Delaware and Illinois, candidates are competing to replace appointed Democratic Senators. In both cases, the winners likely will be seated shortly after the election, potentially threatening a Democratic post-election agenda that might include energy legislation and the defense authorization conference report containing language to repeal the military’s “don’t ask, don’t tell” policy, among other high-profile measures.

The most likely GOP pickup is in Delaware. Republican Rep. Mike Castle is poised to take over for Sen. Ted Kaufman (D), who was appointed to serve as Vice President Joseph Biden’s replacement until a special election is held. Kaufman is not running for the seat, and his appointment expires concurrent with the special election on Nov. 2. The winner will serve through 2014, when Biden’s Senate term expires.

In Illinois, President Barack Obama’s Senate term runs through the end of this year, and his appointed temporary successor, Sen. Roland Burris (D), will be replaced in a special election held concurrently with the Nov. 2 general election. Burris is not running, and state Treasurer Alexi Giannoulias (D) and Rep. Mark Kirk (R) are competing in both the special and general elections in a battle that looks to go down to the wire.

The winners in both states could be sworn in shortly after their electoral outcomes are certified and accepted by the Secretary of the Senate, although Senate Majority Leader Harry Reid (D-Nev.) ultimately decides when the new Members will be seated.

Democrats, with the help of two Independents who caucus with them, currently hold a 59-41 majority. Their advantage could become as narrow as 57-43 in the lame duck. Both Kaufman and Burris were reliable Democratic votes.

Reid is planning a lame-duck session the week of Nov. 15 and more work beginning the week of Nov. 29 after Members have a break for Thanksgiving.

Majority Whip Dick Durbin (D-Ill.) joked when asked last about the post-election math for his party: “Lame-duck session — you skipped September!”

But Durbin acknowledged how precious every vote is in the Senate, even when Democrats seemingly have a comfortable margin. “I sit every day and do the math, simple math,” Durbin said. “You start with 59 you need one, you start with 58 you need two, and it becomes increasingly difficult when you’re short of 60 as I’ve learned since Massachusetts.”

Indeed, Democrats had to switch gears after Republican Scott Brown won the Massachusetts special election in January to replace the late Sen. Edward Kennedy (D). No longer filibuster-proof with only 59 Members, Democrats had to adjust their policies and procedural strategies.

However, such maneuvering becomes increasingly difficult and problematic if Democrats further lose their grip.

“It would be political hari-kari for the Democrat lame-duck agenda to reflect anything other than the message sent by American voters this fall,” a senior GOP aide said.

White House energy adviser Carol Browner suggested Sunday that the Senate could take up some kind of climate change bill during the lame-duck session, after failing to bring such a measure to the floor before adjourning this month. Aides acknowledge the slim possibility that the Senate might use the post-election work stretch to conference on a bill with the House, which passed a comprehensive energy bill last year that largely divided the parties.

Senate Democrats also have to deal with the politically tough issue of extending the expiring tax cuts enacted in 2001 and 2003 under President George W. Bush, a debate that promises to be a battle royal both in committee and on the Senate floor. Also left on the docket: appropriations bills, a continuing resolution or an omnibus spending bill to fund the government, a food safety measure and a small jobs package that Reid hopes to clear in September.

The Senate also is slated to consider the defense authorization bill, which includes language repealing the controversial policy banning openly gay service members. An extensive conference with the House is assured given other political land mines tucked into the bill, suggesting a conference report won’t be ready for consideration until after Election Day.

Commerce Chairman Jay Rockefeller (D-W.Va.) has said he wants to bring a highway safety bill to the floor, and Budget Chairman Kent Conrad (D-N.D.) hopes the bipartisan fiscal commission, which is working on what likely will be a politically explosive deficit-cutting package, has recommendations for the chamber to consider shortly after Nov. 2.

Republicans hope to widen their lame-duck advantage by even more than two seats, but that seems highly unlikely.

West Virginia also has a special election on Nov. 2 to fill the term previously held by the late Sen. Robert Byrd (D-W.Va.), but two-term Gov. Joe Manchin (D) is expected to win handily.

The GOP argues that the temporary term for Sen. Michael Bennet (D-Colo.), appointed to fill the seat vacated by Interior Secretary Ken Salazar, expires immediately after Election Day. However, the Colorado secretary of state’s office maintains the appointment of Bennet, who is running in his state’s primary today for a full six-year term, lasts until Jan. 3.

Republicans undoubtedly will be eager to get their new Members seated as quickly as possible after the Nov. 2 election, although National Republican Senatorial Committee Chairman John Cornyn (Texas) acknowledged it may take some time.

“I can safely conclude it’s not going to be instantaneous,” Cornyn said. “And the questions is when key votes would be held and whether those individuals would be here.”