WASHINGTON (Reuters) - While Congress brawls over how to shrink a $1.6 trillion budget deficit this year, an even tougher battle has begun over cutting popular Social Security retirement benefits to achieve long-term savings. It's unclear whether Congress will be able to agree on how to reform the massive government pension program, which has provided a social safety net for the elderly since 1935. That in part will depend on whether President Barack Obama throws his weight behind a reform effort that politicians of all stripes fear could be full of political risks. While the president has invited a conversation...