The Congressman or Senator can prod USCIS to find out whether the case actually was approved back when the case status online showed it, but they aren't going to research obscure laws and court precedents to find out if the denial is legitimate.
Most US Representatives and Senators have staff members assigned to various constituent issues, including immigration. When a constituent makes an immigration-related request, these staff members contact USCIS. USCIS has a legislative affairs and liaison office that responds to requests from US Representatives and US Senators. The Congressional staffers don't do the research, but they can ask USCIS to look into the case to see what is going on. It can be much more than just prodding to check the status of a case.
Here is a link to how this works with Congressman Jim Moran's office. He represents the 8th District of Virginia.
http://moran.house.gov/casework.shtml
Senator Jim Webb of Virginia offers a similar service for constituents on his site.
My wife and I had to rely on help from the representative representing our district in Massachusetts years ago. How and what Congressional staffers can do for you all depends on how you approach them and how prepared you are to be persistent, organized, and articulate about the specific issue you need help with. They didn't just check status for us.
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