If the issue were really that Ben Nelson has a deep-seated desire to advance a progressive legislative agenda but worries about how it’ll play back home in Nebraska, it would be easy enough for him to decide that the key priorities on which Barack Obama won a national mandate last November all deserve an up or down vote. If he ultimately chose to vote “no” on legislation that he thinks Nebraska voters won’t support, that would be that. You don’t need Nelson’s vote to get to 50.
At the end of the day, though, you don’t erect procedural roadblocks to legislation because you’re playing to public sentiment back home. You use procedural roadblocks when you really don’t want something to pass.
Eh. I'm skeptical. Certainly, a "nay" vote on the substance can help someone like Nelson muddy the waters back home. But it hardly would give him a pass -- the conservative activist groups which would target him know that the procedural vote is the one that matters, and will release the exact same ads lambasting him for his "support".
The fact is that what constitutes "opposing" a bill has changed. This may be a bad thing, but nowadays you're not really "opposing" a bill in the Senate unless you're trying to block it. When Democrats were the ones trying to block GOP bills (like telecom immunity), we weren't going to take a no vote on the merits as a sufficient substitute for filibustering (if the latter was the only way to stop the bill). It's silly to expect conservatives to do otherwise.