How do they know the results without even trying? It takes leadership to pursuade the electorate and opposing politicians. With your view of it, say goodbye to banning partial birth abortions forever.
I think it's pretty simple: think of what Democrats would vote to ban PBA, knowing full well that if the ban passes it WILL be signed into law at the White House. Zell probably comes to our side, maybe one or two others. Not think of how many Republicans vote against the ban: Snowe, Jeffords (at the time), Spector, and probably a few more. It wouldn't pass a simple majority, much less the 60 needed to move the bill through the Senate.
Now that the landscape has changed, there's a chance Bush can jawbone the Senate into passing the law. I'd give him 40-60 at best, even with a 90% approval rating.
Because they have learned over the years, and know what they are doing. Either one of these would be good for you to accomplish.