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McCain tours the Worth and Company factory in Pipersville, Pennsylvania, Monday.

(CNN) – As John McCain prepares to promote the advantages of free trade during a high-profile trip to Colombia and Mexico, a poll released Tuesday suggests the issue may pose a political hurdle for the Arizona senator as the general election campaign heats up.

According to the CNN/Opinion Research Corporation poll, 51 percent of the Americans questioned view foreign trade as a threat to the economy - the first time in a CNN poll that a majority of respondents report holding negative views on free trade.

Read full poll results [PDF]

That compares to only 35 percent of those asked who felt that free trade posed a threat to the economy in 2000, and 48 percent who felt it was a threat in 2006.

Now only four in 10 the Americans questioned say free trade presents an opportunity for economic growth, a sentiment that clearly makes the issue a challenge for McCain, especially in the crucial Rust Belt states most affected by the loss of manufacturing jobs over the last decade.

Election Center: Check out where McCain, Obama stand on trade

"It's possible that John McCain's decision to highlight his free-trade position may wind up losing him some votes among Americans who feel threatened economically by competition from other countries," CNN Polling Director Keating Holland said. "Recent polls in states like Pennsylvania and Michigan have shown McCain well behind Barack Obama. Calling attention to his stand on an issue like trade may be a part of the problems he apparently faces in those Midwestern states."

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