Grinstein told the U.S. Senate Commerce and Transportation Committee a combination of the two airlines would lead to the loss of 10,000 jobs and reduced service for consumers.
"If you believe this merger won't lead to reduced service, you believe in Tinker Bell," he said.
Grinstein also denied Delta was in merger talks with Eagan, Minn.-based Northwest Airlines.
US Airways Chief Executive Doug Parker, sitting at the same witness table, repeated US Airways' pledge all points currently served by Delta or US Airways would be served by the merged airline, Aviation Daily reported.
He said an efficiently run combined carrier could also lower fares and he urged lawmakers to "let the market work."
Parker said the merger of US Airways and America West Airlines was an example of a successful combination.
"Many experts" initially questioned that deal, he said.
"I am proud to say that for our employees, our shareholders and our customers, that 'foolish bet' has paid off handsomely," he said, predicting a strong profit when US Airways announces its 2006 results next week.
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