"The plane landed safely and everyone is OK," the campaign spokesman told AFP, adding that Ann Romney spoke with her husband by telephone when she landed.
Ann Romney had been flying to California on a chartered jet, the campaign said. Her husband was campaigning in the western state of Nevada at the time, and flies to San Francisco later Friday.
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Emergency crews met Mrs Romney's plane on the tarmac in Denver, and Romney spokeswoman Andrea Saul posted a photograph on Twitter showing firemen rushing up the steps of the jet.
"Cause of @AnnDRomney emergency landing, apparently an electrical fire," Saul tweeted.
It is the second plane mishap affecting the Romneys in less than a week in Colorado.
Mitt Romney, who was set to fly last Sunday to the battleground state for a campaign rally that day in the city of Pueblo, was forced to cancel his appearance after a small plane crashed at the airport where Romney was due to land.
On September 8, Romney's campaign plane experienced problems before take-off in Richmond, Virginia, forcing the Republican presidential nominee and his entourage to secure another flight home to Massachusetts.
