By John Whitesides, Political Correspondent

WASHINGTON (Reuters) - In a sharp turnaround, Republican John McCain has opened a 5-point lead on Democrat Barack Obama in the U.S. presidential race and is seen as a stronger manager of the economy, according to a Reuters/Zogby poll released on Wednesday.

McCain leads Obama among likely U.S. voters by 46 percent to 41 percent, wiping out Obama's solid 7-point advantage in July and taking his first lead in the monthly Reuters/Zogby poll.

The reversal follows a month of attacks by McCain, who has questioned Obama's experience, criticized his opposition to most new offshore oil drilling and mocked his overseas trip.

The poll was taken Thursday through Saturday as Obama wrapped up a weeklong vacation in Hawaii that ceded the political spotlight to McCain, who seized on Russia's invasion of Georgia to emphasize his foreign policy views.

"There is no doubt the campaign to discredit Obama is paying off for McCain right now," pollster John Zogby said. "This is a significant ebb for Obama."

McCain now has a 9-point edge, 49 percent to 40 percent, over Obama on the critical question of who would be the best manager of the economy -- an issue nearly half of voters said was their top concern in the November 4 presidential election.

That margin reversed Obama's 4-point edge last month on the economy over McCain, an Arizona senator and former Vietnam prisoner of war who has admitted a lack of economic expertise and shows far greater interest in foreign and military policy.

McCain has been on the offensive against Obama during the last month over energy concerns, with polls showing strong majorities supporting his call for an expansion of offshore oil drilling as gasoline prices hover near $4 a gallon.

Obama had opposed new offshore drilling, but said recently he would support a limited expansion as part of a comprehensive energy program.

That was one of several recent policy shifts for Obama, as he positions himself for the general election battle. But Zogby said the changes could be taking a toll on Obama's support, particularly among Democrats and self-described liberals.

"That hairline difference between nuance and what appears to be flip-flopping is hurting him with liberal voters," Zogby said.

Obama's support among Democrats fell 9 percentage points this month to 74 percent, while McCain has the backing of 81 percent of Republicans. Support for Obama, an Illinois senator, fell 12 percentage points among liberals, with 10 percent of liberals still undecided compared to 9 percent of conservatives.

OBAMA NEEDS TO WORK ON BASE

"Conservatives were supposed to be the bigger problem for McCain," Zogby said. "Obama still has work to do on his base. At this point McCain seems to be doing a better job with his."

The dip in support for Obama, who would be the first black U.S. president, cut across demographic and ideological lines. He slipped among Catholics, born-again Christians, women, independents and younger voters. He retained the support of more than 90 percent of black voters.

"There were no wild swings, there isn't one group that is radically different than last month or even two months ago. It was just a steady decline for Obama across the board," Zogby said.

Obama's support among voters between the ages of 18 and 29, which had been one of his strengths, slipped 12 percentage points to 52 percent. McCain, who will turn 72 next week, was winning 40 percent of younger voters.

"Those are not the numbers Obama needs to win," Zogby said about Americans under 30. The 47-year-old is counting on a strong turnout among young voters, a key bloc of support during his primary battle with New York Sen. Hillary Clinton.

It made little difference when independent candidate Ralph Nader and Libertarian Party candidate Bob Barr, who are both trying to add their names to state ballots.

McCain still held a 5-point edge over Obama, 44 percent to 39 percent, when all four names were included. Barr earned 3 percent and Nader 2 percent.

Most national polls have given Obama a narrow lead over McCain throughout the summer. In the Reuters/Zogby poll, Obama had a 5-point lead in June, shortly after he clinched the Democratic nomination, and an 8-point lead on McCain in May.

The telephone poll of 1,089 likely voters had a margin of error of 3 percentage points.

The poll was taken as both candidates head into their nominating conventions and the announcements of their choices of vice presidential picks. The Democratic convention begins on Monday in Denver, with the Republican convention opening the next Monday, September 1, in St. Paul, Minnesota..

(Editing by Patricia Wilson and Patricia Zengerle)