In the Senate, a single Senator can block legislation with a filibuster. To overcome a filibuster takes a three-fifths vote (60 votes out of 100 Senators). Therefore, for a party to pass whatever legislation it wants, it not only needs to occupy the White House, it also needs to have a majority (218 of 435 votes) in the House of Representatives and 60 votes in the Senate.

Here is a timeline of events related to number of Democrats in the Senate:

-- January 2009: 58 votes (56 Democrats and Bernie Sanders and Joe Lieberman who are registered Independents that caucus with the Democrats)

-- April 28, 2009 Arlen Specter switches parties (59)

-- mid May (between May 9th and 18th) Ted Kennedy leaves Washington (58)

-- May 15 Robert Byrd hospitalized (57)

-- July 7, 2009 Al Franken sworn in after contested election (58)

-- July 21, 2009 Robert Byrd returns to Senate (59)

-- August 25, 2009 Ted Kennedy Dies (still 59)

-- September 25, 2009 Paul Kirk sworn in as Ted Kennedy's replacement (60)

-- February 4, 2010 Scott Brown sworn in

Ted Kennedy had been diagnosed with brain cancer in 2008 and his last vote was on April 27, 2009. If you don't include the time between when Robert Byrd returned to the Senate and when Ted Kennedy dies (since Kennedy left Washington D.C. in mid-May and never returned), Democrats "controlled" the Senate from September 25, 2009 to February 4, 2010 which is 132 days (4 months and 9 days.) If you include that time (from July 21 to August 25) it would add an additional 35 days and would total about 5 1/2 months.

(Also note that the 132 days includes 38 weekend days and the Thanksgiving, Christmas, and New Year holidays.)

References:

http://en.wikipedia.org/wiki/111th_United_States_Congress#Party_summary

http://www.senate.gov/legislative/LIS/roll_call_lists/roll_call_vote_cfm.cfm?congress=111&session;=1&vote;=00059 (filibuster of Stimulus amendment was overcome with 61 votes by getting 3 republican votes Collins, Snowe, and Specter - Feb 7, 2009)

http://bolson.org/gov/us/senate/2009/Kennedy_D_MA.html (last vote by Kennedy)

http://www.capecodonline.com/apps/pbcs.dll/article?AID=/20090509/NEWS/905090329/-1/rss02 (Kennedy still in D.C.)

http://thecaucus.blogs.nytimes.com/2009/05/13/senate-republicans-block-no-2-interior-nominee/ (Kennedy misses filibuster vote)

http://www.cbn.com/cbnnews/politics/2009/May/91-year-old-Sen-Byrd-Hospitalized-/ (Byrd hospitalized)

http://www.youtube.com/watch?v=ZS41vbhIoXg (Kennedy and Byrd out)

http://www.c-spanvideo.org/program/287498-4 (Al Franken swearing in video)

http://www.huffingtonpost.com/2009/07/21/senator-byrd-returns-to-s_n_242051.html (Byrd returns)

http://www.c-span.org/Events/Sen-Paul-Kirk-D-MA-Swearing-In-Ceremony/15385/ (Paul Kirk swearing in video)

http://www.c-span.org/Events/Sen-Scott-Brown-R-MA-Swearing-In-Ceremony/17024/ (Scott Brown swearing in video)

https://en.wikipedia.org/wiki/Filibuster_in_the_United_States_Senate

http://www.rules.senate.gov/public/index.cfm?p=RuleXXII

http://www.foxnews.com/politics/2009/07/ 01/democrats-senate-supermajority-strong-advertised/