Even if they vocally oppose it now, how do we know they will continue to oppose it once in office.

Remember that Obama was pretty vocally opposed to secret government surveillance prior to becoming president.

You make a good point. How can we hold politicians accountable? Perhaps we could vote to shorten terms.

I don't think shorter terms would work. Obama backpedaling on all his initial campaign promises didn't prevent him from getting re-elected. Representatives in the House only have 2 year terms but the highest incumbency rates. I think the main issue is the two-party system. It puts people into this "lesser of two evils" mentality which, surprise, results in some form of evil always persisting.