Immigrant advocates are pressing lawmakers to back legislation they say will help reduce the level of deportations in Massachusetts. Several dozen activists rallied on the steps of the Statehouse on Wednesday in favor of the bill that would instruct local law enforcement agencies not to forward information to federal Immigration and Customs Enforcement officials on illegal immigrants who don’t have serious criminal convictions. Sen. James Eldridge, the bill’s lead Senate sponsor, said the measure is a reaction to the federal Se Communities program, which shares arrestee fingerprints with the FBI and the Department of Homeland Security. Eldridge said most of those deported under the program from Massachusetts had no criminal convictions. He said lower level encounters with police, like traffic stops, shouldn’t end up triggering deportations.