If you are still confused about why many GOP members of the House (and even a few in the Senate) fear being understood to break their pledge against raising taxes, visit the web site for The Campaign for Primary Accountability. This group --a super PAC with significant resources behind it-- targets incumbents who, in its view, ought to be toppled for their indifference to the basic principles on which they were elected.
In the last cycle it went after most notably the Chair of the House Financial Services Committee, Spencer Bachus. It didn't topple the Alabama Republican, but it did exhaust his treasury and expose his record, and the CPA will probably be back around again this cycle to take on Bachus a second time.
A vote to raise the highest rate may or may not be enough to trigger a primary challenge --candor in admitting the breach coupled with serious fiscal conservatism will no doubt protect some incumbents. The worst thing for any sitting House GOPer to do is pretend that any vote to raise taxes isn't a vote to raise taxes just because it prevents tax hikes on most people. Confession of impotence and appeals to the greater good make a lot more sense than arguing "up is down," because candor excuses many political sins.