Why are administration officials and (some) key congressional leaders so determined to reach out to Republicans on health care? It's not necessarily because they want Republican votes. Marc Ambinder explained this in a smart post on Friday:
...the key number here is 60. Not 60 Democrats and Republicans, but 60 Democrats. Or 58 Democrats and two Republicans. ... when the left hears "bipartisanship," they think "Republican votes." Actually, the White House and key Democratic lawmakers want 60 votes. They can push through chunks of health care reform using budget reconciliation rules, but there is a real worry that such reconciled provisions would have no effect because they'd sunset in five years and would face bureaucratic dithering (and even a court challenge.) The White House doesn't want to use reconciliation because they'd get impermanent reform, piecemeal reform, and potentially weak reform.
The issue here is that centrist Democrats like Ben Nelson and Mary Landrieu don't wan't to stand with Democrats against unified Republican opposition, at least if they can avoid it. It may be for the sake of appearances, since come from red-ish states. Or it may be because they're genuinely so conservative. Whatever. The way Obama and his allies can overcome that is by making efforts to woo Republicans; even if those efforts are ultimately unsuccessful, it will make the votes easier on the the likes of Nelson and Landrieu, not to mention the likes of Olympia Snow or Susan Collins.
Of course, that doesn't mean the left should hold its fire. On the contrary, the available evidence suggests strongly that the left should--if anything--push even harder. (I hope somebody is circulatio Jon Chait's new article about primary challenges on Capitol Hill.)
These wavering centrists clearly respond to political pressure. If the pressure from the left is equal to, let alone greater than, the pressure from the right, that's bound to alter their thinking.