WASHINGTON (Reuters) - Representative Paul Ryan has had little immediate impact on the U.S. presidential race since Republican Mitt Romney picked him to be his vice presidential running mate, according to a Reuters/Ipsos poll released on Monday.

Some 51 percent of those surveyed said the decision did not change their opinion of Romney, a former private-equity executive and Massachusetts governor who will face Democratic President Barack Obama in the November 6 election.

Another 26 percent of those surveyed in the online poll said they viewed Romney more favorably after he added the 42-year-old Wisconsin congressman to the ticket on Saturday, while 23 percent said they viewed him less favorably.

The poll of 508 registered voters was conducted for Reuters from August 11 to August 13. The precision of the Reuters/Ipsos online polls is measured using a credibility interval. In this case, the poll has a credibility interval of plus or minus 5.1 percentage points for all respondents.

(Reporting By Andy Sullivan, Editing by Alistair Bell and Doina Chaicu)