As a health reform bill endorsed by President Obama marches toward its first Senate floor vote on Saturday, his opponents stepped up efforts to define the legislation as big-government ambition run amok that will interfere with intimate medical decisions and threaten the pocketbooks of average taxpayers.

A dozen Republican senators spent Friday in an orchestrated, hours-long session of floor speeches, assailing the health-care bill as a vehicle to higher taxes, Medicare cuts and premium increases. Meanwhile, the surprise recommendation this week by a quasi-government panel that women should have fewer mammograms injected a volatile issue into the health debate.

Conservatives on radio and television have seized on the controversial recommendation as an example of the...