WASHINGTON – President Barack Obama is launching the nation's most sweeping effort to curb gun violence in nearly two decades, urging a reluctant Congress to ban military-style assault weapons and high-capacity ammunition magazines like those used in last month's massacre of 20 elementary school children in Newtown, Conn.

The broad package Obama will announce today is expected to include more than a dozen steps the president can take on his own through executive action. Those measures will provide a pathway for skirting opposing lawmakers, but they will be limited in scope, and in some cases, focused simply on enforcing existing laws.

But Congress would have to approve the bans on assault weapons and ammunition magazines holding more than 10 bullets, along with a requirement for universal background checks on gun buyers. Some gun control advocates worry that opposition from Republicans and conservative Democrats, as well as the National Rifle Association, will be too great to overcome.