They can't expel members, but they can certainly kick them out of caucus. That's effectively the same immediate power parties have in a parliament: when someone is removed from caucus they sit as an independent.
The biggest direct difference is that in many parliamentary systems the party has control over who will run under their banner in the next election. In the US, the only power they have is indirect via fundraising.
Really the difference is one of expectation: in the parliamentary system party discipline is just a ground rule that's understood by everybody. If your MP votes for something you don't like they can say "Yeah I didn't like that bill either and I brought up my concerns during our caucus meeting. Ultimately the party decided to support it though" and you don't really think less of them.
That expectation cuts the other way too. If a US Representative got kicked out of caucus over a controversial vote it would be considered shockingly petty. It'd be the major political story for weeks and would tarnish the whole party. In parliament that's just what happens and nobody would expect any different. A semi-recent Canadian that comes to mind:
http://www.cbc.ca/news/canada/nova-scotia/story/2007/06/05/b...
As you mention, they can (and do) withhold committee assignments. Usually this is done more on the sly though rather than as an obvious retaliation. Members that have a reputation for toeing the party line get the nice assignments, the pork project for their district, etc. The "guy none of us can stand but keeps getting elected without our help" just gets to posture for the cameras.
The biggest direct difference is that in many parliamentary systems the party has control over who will run under their banner in the next election. In the US, the only power they have is indirect via fundraising.
Really the difference is one of expectation: in the parliamentary system party discipline is just a ground rule that's understood by everybody. If your MP votes for something you don't like they can say "Yeah I didn't like that bill either and I brought up my concerns during our caucus meeting. Ultimately the party decided to support it though" and you don't really think less of them.
That expectation cuts the other way too. If a US Representative got kicked out of caucus over a controversial vote it would be considered shockingly petty. It'd be the major political story for weeks and would tarnish the whole party. In parliament that's just what happens and nobody would expect any different. A semi-recent Canadian that comes to mind: http://www.cbc.ca/news/canada/nova-scotia/story/2007/06/05/b...
As you mention, they can (and do) withhold committee assignments. Usually this is done more on the sly though rather than as an obvious retaliation. Members that have a reputation for toeing the party line get the nice assignments, the pork project for their district, etc. The "guy none of us can stand but keeps getting elected without our help" just gets to posture for the cameras.