TAMPA, Fla. — For more than 50 years, the Sun Belt — the band of states that extends from Florida to California — has been the philosophical heart and electoral engine of the Republican Party. It was more than just a source of votes. The Sun Belt infused the Republican Party with a frontier spirit: the optimistic, free-ranging embrace of individualism and the disdain for big government and regulation.

From Richard M. Nixon through John McCain, a span of 48 years, every Republican presidential candidate save for Gerald R. Ford and Bob Dole has claimed ties to the Sun Belt. The last Republican president, George W. Bush, made a point of fixing his political compass in Texas once he was done with Yale and Harvard Business School, complete with what many heard as a slightly exaggerated drawl, as had his father, a Connecticut Yankee turned Texas oilman.

Yet as Republicans gather here this week, they are nominating for president a governor of Massachusetts who was born in Michigan and, for vice president, a congressman from Wisconsin. Meanwhile, Sun Belt states that were once reliable parts of the Republican electoral map are turning blue or have turned blue, like California. Only Southern notches of the belt remain. And the sunny symbol of Ronald Reagan in a cowboy hat cutting wood, as good an image of the Sun Belt spirit as there was, has given way to the angrier politics of the Tea Party, which embraces much of the same anti-government message but with a decidedly different tone.

The Sun Belt remains an economic, political and cultural force. But the 40th Republican National Convention is a sign that the Republicans’ grip on it is loosening. The nominations of Mitt Romney and Paul D. Ryan could mark the end of an era.