Bush Veto Forthcoming?

Somehow, President Bush has managed to make it through nearly 6 years as the President without having vetoes a single bill. That in itself is remarkable. The President is now threatening to veto a bill providing for increased funding of stem cell research, and I have two basic questions:

1.) Is the President's veto threat credible, considering he has never vetoed a bill?

2.) Why choose to make a stand on this issue, now?

The legislation must be supported by the Republican leadership in the House and Senate, or it wouldn't have ever been brought up for a vote. Bill Frist, Senate Majority leader, is purportedly a supporter of the bill. This seems a good way to bring negative attention to an issue that I suspect fractures Republicans much more deeply than it fractures Republicans. If I were the Democratic party, I would be thrilled right now that this legislation is set to pass the Senate comfortably, and that the President was going to take such a firm stand on it. If the veto happens, many Republicans will have to make a tough decision on an override vote, if one is forthcoming. I suspect that standing on principle could lose them some support amongst moderate voters. On the other hand, supporting the research could cost them points with the base. Tough call.