Donald Trump reversed course Friday and endorsed House Speaker Paul Ryan for re-election, as he sought to stop his presidential campaign's bleeding after several self-inflicted wounds and relentless criticism from his own Republican Party.

Facing sinking poll numbers and desperate to reset a campaign that has perhaps reached its lowest point of the year, Trump made a bald plea for unity and pledged to work with the very party leaders he had earlier dismissed as Washington's ineffective establishment figures.

"We need unity. We have to win this election," Trump told a rally in Ryan's state of Wisconsin, as he stressed a "big tent" Republican Party is the only way to defeat Democrat Hillary Clinton in November's election.

"In our shared mission to make America great again, I support and endorse our Speaker of the House Paul Ryan," he said, to cheers from the Green Bay crowd. He also announced support for senators John McCain and Kelly Ayotte, two lawmakers facing tough re-election fights.

Meanwhile, a Reuters/Ipsos opinion poll released on Friday showed Hillary Clinton's lead over Trump narrowed to less than 3 percentage points, down from nearly eight points on Monday.

About 42 percent of likely voters favored Clinton, to Trump's 39 percent, according to the July 31-Aug. 4 online poll of 1,154 likely voters. The poll had a credibility interval of plus or minus 3 percentage points, meaning that the results suggest the race is roughly even.