Republican Sen. and Democratic Sens. and came off the campaign trail to talk with Brown in a series of sessions.
Brown succeeded Tony Blair last June and is on his second visit to the United States - and to see President Bush - as prime minister.
"We think it's probably a wise move by the prime minister to get to know one of the individuals who will be elected president a year from now," White House spokesman Tony Fratto said, acknowledging the next chapter in U.S.-British relations. "It makes sense."
Brown met one-on-one with Obama, then Clinton, then McCain in 45-minute intervals spanning three hours at the British ambassador's residence in Washington.
It was Obama's first meeting with Brown. McCain met Brown last month during a visit to London.
The talks gave McCain, Clinton and Obama the opportunity to appear presidential alongside a foreign leader; cameras captured each of them with Brown.
A member of Britain's Labour Party, Brown praised all three in TV interviews and predicted that the link between Europe and the United States would grow stronger in coming years.
"I feel I can bring Europe and America closer together for the future," Brown said. "That will be to the advantage of all of us, to deal with economic problems, climate change and help make for a more peaceful world in the future. I see huge opportunities in the next few years for Europe and America to work more closely together."
With new leadership already in Britain and upcoming in the United States, relations between the two countries are poised for change.
Mr. Bush and Blair had an extraordinarily steadfast bond, strengthened in part by their unwavering support for the Iraq war. However, Bush is highly unpopular in Britain and Blair's political fortunes soured because of the friendship.
In contrast with his predecessor, Brown has taken a more cautious approach with Bush and the relationship grew tense when Britain decided to draw down its troops in Iraq. He has said a plan to reduce British troop numbers from about 4,000 to 2,500 would remain on hold; it was delayed after a recent spike in violence in the southern port city of Basra.
Of the presidential candidates, McCain is a staunch backer of a continued U.S. military presence in Iraq, while Clinton and Obama have called for withdrawing troops.
As Obama and Brown met, President Bush's motorcade passed the front of the British ambassador's residence.