DENVER  President Obama has not ruled out a second stimulus package, his press secretary, Robert Gibbs, said on Tuesday, just before Mr. Obama signed his $787 billion recovery package into law with a statement that it would “set our economy on a firmer foundation.”

The president said he would not pretend “that today marks the end of our economic problems.”

“Nor does it constitute all of what we have to do to turn our economy around,” Mr. Obama said at the signing ceremony in the Denver Museum of Nature and Science. “But today does mark the beginning of the end, the beginning of what we need to do to create jobs for Americans scrambling in the way of layoffs.”

Mr. Gibbs, speaking to reporters aboard Air Force One on the way to Denver, said, “I think the president is going to do what’s necessary to grow this economy.” While “there are no particular plans at this point for a second stimulus package,” he added, “I wouldn’t foreclose it.”

Mr. Obama began the first leg of a two-day trip, using the museum ceremony to spotlight the bill’s clean-energy provisions. The president will also visit Phoenix, where he will unveil his new housing plan on Wednesday.