WASHINGTON — 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, conservative groups and liberal ones. The lawmakers argued that this time, thanks to that broad-based support, immigration overhaul 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. "I am confident this issue will not fall victim to the usual partisan deadlock."

Support for the bill is already being put to the test as conservatives grow more vocal in opposition. Two Republican senators held a dueling news conference with law enforcement officials to bash the bill's security provisions, and several conservative bloggers seized on one provision of the legislation to falsely claim that it would allow people here illegally to get free cellphones.