Brown declined to endorse one candidate over another but said, "What I was convinced of, after talking to each of them..is that the relationship between America and Britain will remain strong, remain steadfast."
Brown made clear earlier that he hopes for an improved partnership between Europe and Washington under the next administration when divisions over Iraq come to an end.
While Brown is closer on the political spectrum to the two Democrats than to McCain, Bush's fellow Republican, all three have taken stands that could win favor with the head of Britain's center-left Labour Party.
They have signaled, for example, they would all do more to fight global warming and revamp the Bush administration's internationally criticized treatment of terrorism suspects.
McCain's sharpest difference with his Democratic rivals is on Iraq. He backs Bush's strategy of maintaining a heavy U.S. troop presence. Clinton and Obama, battling for the right to face McCain in the election, want a withdrawal timetable.
For his part, Brown has stirred unease in Washington over Britain's phased troop drawdown in Iraq, where Blair had bucked British public opinion in joining the 2003 U.S.-led invasion.
After their meeting, Obama, who would be the first black U.S. president, said he and Brown discussed issues in common such as Iraq and Afghanistan, the global economy and Africa.
Campaigning in Pennsylvania, Clinton, who would be America's first woman president, said she and Brown talked about the environment. She said Britain had done a lot more against climate change than the Bush administration.
Reporters overheard Brown telling McCain it seemed like the U.S. presidential campaign had been going on for a year. The Arizona senator said it had and quipped that it made one wish sometimes for the British electoral system.
Brown's unusual talks with the candidates perked up U.S. media interest in his three-day tour, which has been largely eclipsed by Pope Benedict's visit this week.
But signs of a warming trend with Bush may not help Brown's poll ratings at home, which have been dropping faster than any British leader since before World War Two. The 57-year-old Scot is mindful that Blair's closeness to Bush angered many Britons and contributed to his decision to step down early.
Brown, like Bush, has seen his economic stewardship come under heavy fire.
Bush, meanwhile, is struggling to stay relevant but is having a harder time swaying world leaders looking to whomever will succeed him in January 2009.
