Hillary Clinton scored a hefty win in Sunday's Puerto Rico primary but Barack Obama looked to have the Democrats' White House nod within his grasp after a fractious deal on Florida and Michigan.

The Caribbean US territory of four million people held the third-last nominating contest heading to Tuesday's climax of the five-month primary season in Montana and South Dakota.

The CNN, Fox News and MSNBC networks said Puerto Ricans had voted decisively for Clinton, but MSNBC said turnout was poor at about 10 percent.

Puerto Ricans, who are US citizens but have no representation in Congress, can take part in party contests but do not have the right to vote in November's presidential election.

Thousands staged a colourful protest in the capital San Juan to agitate for Puerto Rico's independence from US control, many wearing Obama and Clinton masks to condemn the primary as an irrelevant "circus."

As the former first lady prepared for a victory rally in San Juan, her campaign chairman Terry McAuliffe told ABC News: "We're not going anywhere until someone has the magic number to be the nominee of the party."

McAuliffe and other Clinton aides pressed anew the argument that the New York senator leads in the popular vote and would "easily" beat Republican John McCain in November.

But the Democratic Party's decision Saturday to reinstate delegates from Michigan and Florida, with their voting power halved, put Obama two giant strides closer to making history as the first black presidential nominee.

Democratic National Committee chairman Howard Dean said the outcome decided by the party's rules and bylaws committee was "fair," and backed even by pro-Clinton state leaders in Florida and Michigan.

"We think this is the beginning of the healing of the party," he said on CNN, despite screams of outrage from Clinton supporters at the committee meeting, with some threatening to vote for McCain instead of Obama.

Obama's communications director Robert Gibbs predicted that "sometime this week, we'll probably have a nominee for the Democratic Party and then we can get to the need to bring change to this country."

"If it's not Tuesday, I think it will be fairly soon," he told Fox.

The Florida and Michigan compromise saw delegates apportioned to both candidates, scuppering Clinton's hopes of making a significant dent in her rival's lead in the race to take on McCain.

The two states had broken party rules by holding their primary votes in January. Clinton won both unofficial contests, though neither candidate campaigned in Florida, and Obama was not on the ballot in Michigan.

The compromise moved the finish line for the Democratic nominating contest up to 2,118 delegates, with Clinton gaining a net 24 delegates from the two-state deal struck at the stormy meeting in a Washington hotel.

That left Obama just 66 delegates short of the target, and he was expected to get about half the total of 86 pledged delegates on offer in Puerto Rico, Montana and South Dakota.

There are also nearly 200 "superdelegates" or top party officials who have yet to declare their allegiance.

Top Clinton aide Harold Ickes complained the party had "hijacked" four delegates from Clinton in Michigan, and said the campaign may challenge the ruling ahead of the party's August nominating convention.

Obama said he believed in the patriotism of Senator Clinton and her husband, former president Bill Clinton, and their loyalty to the Democratic cause.

"They love the Democratic Party. I think they deeply believe that Democrats need to win in November," he said late Saturday. "And so I trust that they're going to do the right thing."

On a day of drama Saturday, Obama also announced he would quit his Chicago church, which has seen preachers fire off a string of racial rhetoric that rocked his campaign.

"This is not a decision I come to lightly, and frankly it is one I make with some sadness," Obama said.