In 2005, Democrats and Republicans on Capitol Hill joined together to raise a firestorm of criticism over Chinese state-owned energy giant Cnooc Ltd.'s $18.5 billion bid to buy U.S. refiner Unocal Corp.

Despite Cnooc's efforts, including the hiring of Washington lobbying and media consultants, the transaction unraveled under congressional pressure, with lawmakers of all stripes arguing that it "threatened to impair" U.S. national and energy security. Unocal was later acquired by Chevron Corp. (US:CVX) and Cnooc deal-makers returned to Beijing with bruised egos and scrapped plans.

Flash forward to 2012. After seven years of quietly making small North American energy purchases and improving relations with key policy makers, Cnooc (US:CEO) has jumped back into the big-time, politically-charged North American deal-making arena with what would be its largest foreign acquisition to-date, a proposed purchase of Calgary, Canada-based Nexen Inc. (CA:NXY) (US:NXY) for $15.1 billion.

Burned by the Unocal failure, Cnooc is treading more carefully this time around, seeking to buy a less well-known but equally strategically important company in a deal that observers expect will pass regulatory muster.

The target, Nexen, has a substantial operating business in the Alberta oil sands; valuable drilling activity in the U.S. portion of the Gulf of Mexico and premium assets in the North Sea located between the U.K. and mainland Europe. The U.S., Canadian and U.K. governments are reviewing the transaction, but regulatory observers believe their interest in the stable investment China brings will outweigh any energy security or trade reciprocity concerns raised about the deal.

"It's been a seven-year courtship since Unocal and they've acted well," said Pierre Fournier, analyst at National Bank Financial in Montreal. "A much bigger acquisition like Suncor that owns a big chunk of U.S. oil would be a different game but this is a diversified company with holdings around the world, not raising many concerned eyebrows, especially since everybody needs the kinds of long-term patient capital China provides."

Some in Ottawa and Washington have raised concerns, but criticism has been muted compared with the Unocal debacle. For one thing, the critics that have emerged haven't called for immediate blockage. By contrast, in 2005, the House of Representatives, with large bipartisan majorities, passed two protest bills including a measure that sought to cut off funding for the acquisition to proceed.

Instead, with Nexen, four U.S. lawmakers — so far — have expressed concern, including longtime China critics, Sens. Charles Schumer and James Inhofe.