As Greg Mankiw points out, taxing health benefits is supported by William Gale and by Jeffrey Frankel, two economists more likely to be found in a Democratic than a Republican administration. They in turn cite Jason Furman, who is also more likely to be found in a Democratic administration.
On the other hand, James Klein and John Sweeney are against taxing health benefits. Klein is a lobbyist for health benefits providers and Sweeney is head of the AFL-CIO. My guess is that Klein and Sweeney can deliver a lot more votes than Gale and Frankel.