By not bringing the Senate bill up for a vote, Republicans are revealing their true goals. Strictly considering the proper tax rate for the bottom 98% of taxpayers, of course Republicans would vote for the lower rates. If they wanted to do so, they could pass the tax cut, Obama would sign it into law, and then they could argue the merits of tax cuts for the wealthy and of spending cuts. But instead they choose to say that if the rich don't get their tax cuts, then the GOP will refuse to let the middle class keep theirs. It clearly shows the relative values placed by Republicans on each group's tax treatment.
There was a peculiar discussion of this issue on CNBC today involving billionaire investor Wilbur Ross. Ross said "The trade-off the president proposed is not a trade-off. To say give me what I want, which is the middle-class relief, and I'll give you a framework for negotiations, that's a sucker's game.”
How is middle-class tax relief suddenly something Republicans have to begrudgingly, and with negotiations/concessions, give to Democrats? They should be happy that their opponents have come around to their side on this one, but instead they're demanding concessions before they'll vote for the things they supposedly support. But they (correctly) realize that a tax cut for the wealthy would never go through on its own, so they're forced to use whatever leverage they have to force the lower top tax rates through. One of the few sources of leverage they have is the expiration of the Bush tax rates for the middle class. They are willing to risk middle class tax cuts to try to get tax cuts for the rich.