"That would be an extremely strong signal for the Obama administration," Hedegaard, the EU Commissioner for Climate Action, told reporters in a briefing in Washington.
Hedegaard has been visiting lawmakers, administration and World Bank officials as well as other groups in Washington and Boston this week.
She is due to meet with Democrats, including Representative Henry Waxman of California and Senator Sheldon Whitehouse of Rhode Island, two of the most vocal supporters of climate legislation in Congress, as well as US envoy for climate change Todd Stern and White House economic adviser Michael Froman.
Hedegaard said rejecting the controversial pipeline, which if completed will transport 830,000 barrels per day of heavy crude oil from Alberta, Canada, to oil refineries in Texas, would show that the United States would "avoid doing something" that could contribute significantly to climate change.
She also said the EU will stick to its plan to label fuel from Canada's tar sands deposits as "highly polluting," deterring EU refiners bound by strict environmental rules.
Canada's Natural Resources Minister said earlier on Thursday that he is "cautiously optimistic" that TransCanada Corp's proposed pipeline will be approved.
US officials say they expect the government to make a final decision on Keystone by the middle of the year.
Airline ETS challenge
Beyond Keystone another test for the administration, Hedegaard said, will be how it engages in negotiations under the United Nations' civil aviation body to devise a global framework to curb emissions from the global aviation sector.
Negotiators will meet again in March, in talks sponsored by the UN's International Civil Aviation Organization.
The group is under pressure to develop a plan by September, after which time the EU will revive a mothballed law that would force all airlines to pay for each tonne of carbon dioxide they emit on flights landing in or departing from the EU.
The threat of that law in 2012 stirred fears of a global trade war. The United States, China, India and Russia all lobbied fiercely against it.
Hedegaard said that she feels progress will be made in the new round of global aviation talks and she expects the United States and the EU to find common ground on aviation emissions.
She added, however, that an early proposal that has been offered by the US - to curb emissions over countries' airspace but exclude emissions from time spent over international waters - would be inadequate because it only covers a "small proportion of the global emissions related to flying."
