The Democrats have won the national popular vote in five of the past six presidential elections. Powered by support from the swelling population of nonwhite voters, Democratic nominees have flourished by repeatedly winning a phalanx of states along the east and west coasts and through the upper Midwest.

If Mrs. Clinton can hold the 18 bulwark states in those areas and the District of Columbia, she would have 242 electoral votes. That would leave her just 28 short of the needed 270, with multiple possibilities for finding them.

But as any sports fan or stock market investor can attest, straight-line projections from the recent past can prove unreliable. As Mr. Clinton showed in 1992, demographic trends, exceptional candidates and disruptive forces can tip political outcomes unexpectedly.

The 1992 campaign began with signs that the Republican juggernaut had weakened.

Four years earlier, the first President George Bush had kept the electoral jackpot of California in the Republican column, but by less than four percentage points. He won another Republican stronghold, Illinois, by just two percentage points.

Then the 1990-91 economic recession badly eroded Mr. Bush’s standing. Under fire from both Mr. Clinton and the feisty independent Ross Perot, he lost both California and Illinois decisively in 1992, as well as New Jersey, which he had carried by a double-digit margin in 1988.