By Steve Holland

FAIRFIELD, Connecticut (Reuters) - Republican front-runner John McCain on Sunday admitted to nervousness about his chances on Super Tuesday but also was thinking about a general election battle against the Democrats.

Ahead of Republican nominating contests in 21 states on Tuesday, McCain has a sizable lead in many of those states and could emerge that day as the presumptive Republican nominee.

After an up-and-down year, he could not help but rap his knuckles on the table in his campaign bus, a wish for good luck by the superstitious McCain.

"I'm very nervous about it because, you know, I've seen this movie before," he told reporters on his bus. "My job is to keep our expectation levels down and frankly not raise expectations to a degree that we could suffer some setback."

It has been a long road to the high ground for the Arizona senator, who at 71 would be the oldest person ever elected to a first presidential term.

His political obituary was being written last summer when his campaign ran out of money and he had to dump staff and scale down operations.

Now he is on the cusp of a big comeback, and over the past couple of days he has alternately veered between exuding confidence to experiencing anxiety about whether he can actually pull it off.

"There have been so many ups and downs in this campaign that for us to think that we have a smooth path to the nomination I think would be contradictory to the history of this campaign," he said.