WASHINGTON (Reuters) - Gun control activists, health care advocates and law enforcement groups geared up on Tuesday for a last-ditch effort to prevent a 1994 ban on assault weapons from expiring next week, but even its most ardent backers acknowledge the drive is all but futile. But the influential National Rifle Association gun lobby, meanwhile, said it would "not take anything for granted" as it works to send the ban into oblivion. Ban advocates called on President Bush (news - web sites) to intervene and get Congress to act. But Bush, who in his 2000 campaign promised to sign legislation,...