Rick Santorum’s surge in national polls is prompting Mitt Romney to begin talking more explicitly about his own conservative social views, in an attempt to blunt Santorum’s appeal to the Republican base. Romney had hoped to stay focused squarely on his economic message and downplay social issues that his advisers believe did not play to his strengths as a onetime businessman, Massachusetts governor, and Winter Olympics chief. But this week Romney inserted a new line into his stump speech: “We were able to enforce, I think it was a 1913 law, that kept Massachusetts from becoming the Las Vegas of same-sex marriage.’’