Because as elucidator said, Democrats are not unified. But so what, neither were Republicans.
If you really look closely you find the Dems and Reps are not that far apart on most issues.
Also remember the Senate is not based on population but on states. Wyoming and Vermont, very small states have as much power as California and Texas.
So it doesn’t matter what party the Senators of VT and WY belong to, they use their “power” to bargain.
For instance, no one cares about a dam in Wyoming except the 100,000 people in Wyoming it’ll effect. A 100,000 people in WY is about 1/5 the state population so to a Senator in WY he has to get that dam built.
But who cares about a 100,000 people. Well if Mr Republican Senator in Wyoming can go to Mr Democratic Senator in California (where each state is equal), he can say, "Support money for my dam and I’ll give you something CA wants)
See how it works
A Republican Senator can’t simply say “I voted on party lines.” The electorate would say “You’re first duty is to the people of the state, not your political party.” And if the electorate wouldn’t say it, at the next election his opponent would say that.
Finally remember Obama barely won the nomination. He got in basically 'cause “Oprah thought he was cute.” Polls indicate in a lot of states the fact he didn’t vote for the Iraq War was the sole reason people voted for him over Hillary.
The Clinton by contrast worked their way as Democratic Party players, so they still hold much influence.
Mr Obama is an upstart or outsider to Washington DC. He has stated the reason he ran for president was because he was tired of the Washington system which prevented him as a Senator from getting anything done.
That’s a slap to those who work within the system. I’m not saying he’s wrong, but it’s like at a workplace, lots of unpleasent people work with you. Even if you’re right and go over their head, when that is over you still have to work with these people, who are now going to be openly hostile to you.
Lastly Senators and Representatives have no term limits. Many are there for decades. Mr Obama will be out by 2012, maybe sooner, and those Congressmen are going to have to work with others long after Mr Obama is gone.
So it pays not to bully your way in and burn all your bridges.
So a Congressman is viewing his priorites as:
- His own job and career. (Maybe he wants to president someday)
- His own state, to keep him re-electec
- His own fundrasiers, (Got to keep them happy, a run for the US Senate, successful or not is estimated to run about 25 million. So even if the voters love you, if they won’t cough up big time you can’t win)
- Then the party.
And note “then the party.” Look at the Clintons, played by all the rules, always helped Democrats, totally went within the Democratic Party rules and system. Hillary had the nomination wrapped up. Then an upstart like Mr Obama came along and took it away.
So looking at it like that loyalty to your party is pretty low on the list, since the party won’t back you when the going gets tough