If you're a U.S. representative, here's one potential metric for measuring how safe you are: If your opponent doesn't live in your district. Fine tune the metric by noting how many miles from the district your opponent lives.
In the case (so far anyway) of Representative Earl Blumenauer, Democrat representing the district that includes most of Portland, that would yield a pretty strong number. He has an opponent, the same one in fact who ran against him in 2008. But Republican Delia Lopez lives about 150 miles from District 3, which Blumenauer represents. Lopez, from Oakland, is personable and might otherwise be a solid candidate for something. But the distance issue is kind of a problem.
You can do that: To run for a U.S. House seat, you only have to live in the same state, not necessarily in the district. But it doesn't often happen (has it ever?) that such an out of district candidate wins, or even comes close.
Math people: Get to work on the metric.