Four Democratic and four Republican senators formally unveiled a sweeping immigration bill Thursday at a news conference attended by traditional opponents from big business and labor, and conservative and liberal groups. The lawmakers argued that this time, thanks to that broad-based support, immigration legislation can succeed in Congress. “Powerful outside forces have helped defeat certain other initiatives in Washington, but on immigration, the opposite is proving true,” Sen. Chuck Schumer, D-N.Y., said a day after senators under intense lobbying pressure blocked a major gun control package.