With the U.S. House of Representatives increasingly likely to pass a bill this week that would expand the use of embryonic stem cells, President George W. Bush may soon confront one of the thorniest issues of his second term : whether to veto a bill supported by influential members of his own party. Supporters of looser restrictions on stem-cell research say they are confident they have the 218 votes necessary to pass legislation that would allow taxpayer dollars to be used for research on embryos that are just a few days old, stored in fertility- clinic freezes and donated by couples who don't need them.