HAMPTON, N.H. — For much of this year, Mitt Romney has laid low, seeking to reap the benefits of being the presumed front-runner in the race for the Republican presidential nomination (strong fund-raising, positive polling) without suffering the downsides (intensive news media scrutiny, endless shelling from rivals).

But as Mr. Romney has finally started to campaign in earnest over the past couple of weeks, the dangers of his perceived leading status are coming into view, with a more intense focus on his every word. And in the past few days, it has revolved around three words in particular: “things are worse.”