Barack Obama jumped to his biggest lead since late July in public opinion polls, after his televised speech to a jam-packed Denver football stadium and his Democratic National Convention reconciliation with rival Hillary Clinton.

How long Obama holds the lead is open to question, as voters react to John McCain's surprise selection Friday of Alaska Gov. Sarah Palin for his running mate and Republicans begin their nominating convention Monday in St. Paul, Minn.

Obama leads McCain 49-41 percent in the most recent Gallup Poll daily tracking survey, which measured voter sentiment during a three-day period ending Aug. 28. The presidential contenders had been tied at 45 percent in the last Gallup tracking results before the Democrats started their Colorado convention.

The 8 percentage-point lead almost matches Obama's biggest margin of the campaign, a 9-point bulge in tracking polls conducted July 24-26, Gallup said. His Denver speech on Aug. 28 attracted 38.4 million television viewers, 57 percent more than the audience in 2004, when Massachusetts Sen. John Kerry accepted the party's presidential nomination, according to Nielsen Co.

The Illinois Democratic senator jumped to a 47-43 percent advantage over McCain in Rasmussen Reports daily tracking poll results through Friday, the biggest margin in that poll since late July, after Obama's speech to 200,000 people in Berlin. Including undecided voters who say they're leaning toward one candidate or another, Obama leads 49-45 percent, Rasmussen said.

Most voters had a good impression of Palin, the 44-year-old first-term Alaska governor who beat out more conventional, experienced Republicans to become McCain's vice presidential pick, according to the Rasmussen survey.

Palin was viewed favorably by 53 percent of respondents. Before her selection, more than two-thirds of Americans surveyed had said they didn't know enough about the Alaska politician to form an opinion. The Democratic vice presidential nominee, Delaware Sen. Joe Biden, drew a favorable opinion from 48 percent of those polled.

While making a good first impression, Palin didn't make an immediate impact on presidential preferences: Thirty-five percent of those surveyed by Rasmussen said her choice made them more likely to vote for McCain, while 33 percent said they were less likely to back the veteran Arizona senator.

Forty-eight percent of women had a favorable impression of Palin, including 80 percent of Republican women and 23 percent of Democratic women, according to the Rasmussen survey.

Gallup's tracking poll interviews at least 1,000 U.S. adults each day, and reflects combined data from the most recent three days of polling. The latest survey included responses from 2,727 registered voters, and has an error margin of plus-or- minus 2 percentage points.

Rasmussen surveys 1,000 likely voters each night, and combines three days of polling in each tracking result. Its poll also has a plus-or-minus 2 percent error margin.