WASHINGTON -- More than half of U.S. adults walked away from last month's Republican National Convention less likely to vote for Donald Trump, the first time in Gallup's polling that a political convention peeled support away from a presidential nominee.

As for former U.S. Secretary of State Hillary Clinton, the first woman nominated for president by a major U.S. political party, 45 percent said they were more likely to support her and 41 percent less likely.

"The Democratic Party's convention left a considerably more positive impression on the American public than the Republican Party's convention," Gallup's Jeffrey M. Jones said in his analysis.

A majority of American adults who paid attention to the Republicans' four-day gathering in Cleveland, 51 percent, said they would be less likely to support Trump. Just 36 percent said they would be more likely to back the billionaire businessman making his first try for public office.

That's a deficit of 15 percentage points, the only time since Gallup began asking the question in 1984 that a candidate lost support coming out of the convention.

A greater percentage of Americans also thought Trump's acceptance speech was poor or terrible rather than good or excellent, the first time in Gallup's polling that a nominee's convention address received a net negative rating.

Clinton's 4-point gain coming out of the Democratic convention in Philadelphia, was the smallest among that party's nominees in polling dating to 1984 -- President Barack Obama recorded a 5-point gain four years ago -- though higher than two Republicans, former Massachusetts Gov. Mitt Romney in 2012 (2 points) and President George W. Bush in 2004 (3 points).

The biggest gain, 45 percentage points, was recorded by Clinton's husband, Bill Clinton, in 1992. Republican George W. Bush had a 17-point increase in 2000. Both Clinton and Bush were elected to the first of two terms.

Americans also thought more highly of Clinton's acceptance speech than Trump's. For Clinton, 44 percent called her address good or excellent while 20 percent said it was poor or terrible.

Trump's speech was viewed as good or excellent by 35 percent and poor of terrible by 36 percent. That was the first time in Gallup polls dating to 1996 that a convention acceptance speech garnered negative reviews.

Romney's 2002 acceptance speech was viewed as good or excellent by 38 percent, but only 16 percent said it was poor or terrible.

The polls of around 1,000 adults were conducted July 23-24 for the Republican convention and July 29-30 for the Democratic convention and had margins of error of 4 percentage points.

Jonathan D. Salant may be reached at jsalant@njadvancemedia.com. Follow him on Twitter @JDSalant. Find NJ.com Politics on Facebook