More than half of US adults used the Web to engage in the US presidential election and supporters of Barack Obama were considerably more active online than those of John McCain, according to a study released on Wednesday.

"The 2008 election was the first in which more than half the voting-age population used the Internet for political purposes," said the Pew Research Center's Internet and American Life Project, which conducted the study.

Fifty-five percent of all voting-age US adults -- and 74 percent of all Internet users -- said they went online for news and information about the election or to communicate with others about the race, the study found.

The percentage of Americans relying on the Web as a major source of campaign news more than doubled over the 2000 election -- from 11 percent to 26 percent, Pew said.

Social networking sites like Facebook and MySpace and video-sharing sites like YouTube played a key role, it said, as voters went online to share their views and try to mobilize others to their cause.

"Voters in 2008 were not just passive followers of the political process," said Aaron Smith, a research specialist at Pew and author of the report.

"They used a wide range of digital tools and technologies to get involved in the race, to harness their creativity in support of their chosen candidate, and to join forces with others who shared their same political goals."

While more Americans went online for political purposes than ever before, supporters of Democratic candidate Obama were more active on the Internet than those of his Republican rival McCain, the study found.

Twenty percent of Obama supporters shared political content online, compared with 16 percent of McCain backers, and 18 percent of Obama partisans signed up to receive automatic updates about the election compared with nine percent of McCain supporters.

Obama supporters were much more likely to contribute money online, with 15 percent of Obama supporters saying they did so compared with only six percent of McCain backers.

"The 2008 elections saw the role of the Internet in politics increase and it witnessed the emergence of a unique group of online political activists," said Smith.

"Compared with other Internet users, these individuals delve more deeply into the political news of the day, and take part in a much wider range of online political activities."

The study was based on a survey of 2,254 adults conducted between November 20 and December 4. It has a margin of error of plus or minus two percentage points.