WASHINGTON - The growing oil slick in the Gulf of Mexico is not only an environmental danger but also a widening political headache for President Obama, who just last month agreed to open vast stretches of the U.S. coastline to oil drilling.

The spill threatens to highlight the environmental risk of offshore exploration -- a risk that critics have long warned about and that Obama tried to downplay when he announced the expansion of drilling off the Atlantic Coast, in Alaska and in the Gulf of Mexico.

White House officials said Thursday that the expanded drilling plan, which is entering a public comment phase, could undergo modifications as a result of the oil spill and possible public reaction to it. They said the same applies to the floundering Senate climate bill, which includes a provision to expand drilling in hopes of wooing Republican support.

White House press secretary Robert Gibbs said it would be impossible to say what effect the spill will have on the drilling plan until investigators determined the cause of the explosion and subsequent leak.

But top energy and climate adviser Carol Browner said "this will become part of the debate" over drilling. "That goes without saying."

The statements were likely to upset Republicans, many of whom criticized Obama's drilling plan as too little, and to please environmentalists who opposed the plan.

Call to halt drilling new wells

The accident in the Gulf may provide more firepower to those critics on the left, who have for years lobbied presidents and Congress to keep in place federal moratoriums on further offshore exploration. Those moratoriums have now expired.

Some lawmakers have already called for congressional inquiry into the regulation of offshore drilling.