The president may only resign his office and then the president of the Senate assumes the presidency temporarily.

VOA: standard.2009.03.20

The last time Democrats had control of the Senate was in 1994 -- except for a period of 17 days earlier this year when Vice President Al Gore held the tie-breaking vote as president of the evenly divided Senate until the new Bush administration took office.

CNN: In the Senate, it's the day of the Democrats