As several leading Republican presidential candidates embrace a flat tax as a core campaign position, one contender stands out in not doing so: Mitt Romney, who has a long record of criticizing such plans and famously derided Steve Forbes’s 1996 proposal as a “tax cut for fat cats.”

Lately, though, his tone has been more positive. “I love a flat tax,” he said in August.

Flat-tax plans have come and gone before, and analysts note that they have tended to lose support once they come under scrutiny. But Mr. Romney’s support of the concept of a flat tax underscores the tightrope he is walking as taxes become a larger focus of the Republican presidential race and he faces rivals’ accusations of inconsistency on the issues…

“His problem is that people don’t have confidence that they know what he believes in, and I think there is a pretty good reason for that,” said Chris Chocola, a Republican former congressman from Indiana who is president of the Club for Growth.

Grover Norquist, head of Americans for Tax Reform, says he sees Mr. Romney’s shift as part of a broader consensus among the field to move toward a flatter tax structure. But with that movement gaining strength in the Republican Party, he said, Mr. Romney’s past critical comments about flat taxes are “awkward” and “a little tough to explain.”