Sixty-eight Nobel Prize winners in the science fields, including the two Americans who won this year’s chemistry prize, have signed a letter endorsing Mr. Obama over his Republican rival, Mitt Romney.

“President Obama understands the key role science has played in building a prosperous America,” the laureates wrote in a letter that was released on Wednesday. Mr. Obama “has championed investment in science and technology research that is the engine of our economy.”

The signers said that Mr. Romney, by contrast, “supports a budget that, if implemented, would devastate a long tradition of support for public research and investment in science at a time when this country’s future depends, as never before, on innovation.

In September, 2008, 61 American winners of a science Nobel — in medicine, physics or chemistry — signed a similar letter endorsing Mr. Obama over Senator John McCain, a number that rose to 76 by the end of October.

That October, Martin Chalfie, a professor of biological sciences at Columbia, was awakened by a phone call from Stockholm that he was one of the winners of the Nobel Prize in Chemistry. He had heard of the letter endorsing Mr. Obama and called a friend to make sure that he was added to the list of signatories.

“That was actually the very first thing that I did as a Nobel laureate,” Dr. Chalfie said. “Scientists should stand up and talk about what they feel is important for the country in terms of the long-term support research."