I've never done this before, but I thought this post from Miami Herald political columnist Joy Reid was so good, I'd link it here. I've argued this point before, but never could have put it so well as Reid does here.
Barack Obama and the myth of the progressive ‘majorities’
...Despite the myth-making on the left, Democrats actually held their tenuous 60-vote majority for only five months in 2009: from July of that year, when Al Franken was finally sworn in after winning the recount against Norm Coleman, through November 2009, when Democrats lost Barack Obama’s old Senate seat in Illinois to Mark Kirk. Then in a special election the following January, Scott Brown won Teddy Kennedy’s old seat, and was sworn in on February 4th.Could Barack Obama have somehow rammed through the entire progressive wish list in five months? I find it hard to see how, given the unreliability of the blue dog Senators.