SAN JUAN, Puerto Rico -- Hillary Clinton's convincing win in Puerto Rico's Democratic presidential primary on Sunday is adding fuel to her claims that the popular vote total should give superdelegates the guidance they need to select her as the nominee.

With 100 percent of the vote counted, Clinton received 68 percent to Barack Obama's 32 percent, winning by more than 140,000 votes.

But lower-than-expected turnout could hamper Clinton's argument that she can attract enthusiastic general election voters and is the better candidate than Obama in the fall race against presumptive Republican nominee John McCain.

"I am grateful for this show of overwhelming support," Clinton told supporters at a rally in the capital city. "You are joining millions of people across the United States -- more than 17.6 million plus the votes we received today. ... When the voting concludes on Tuesday, neither Senator Obama nor I will have the number of delegates to be the nominee. I will lead the popular vote. He will maintain a slight lead in the delegate count.

"The decision will fall on the shoulders of those leaders in our party empowered by the rules to vote at the Democratic convention. I do not envy the decision you must make, but the decision has to be made, and in the final assessment I ask you to answer these questions. Which candidate best represents the will of the people who voted in this historic primary? Which candidate is best able to lead us to victory in November? And which candidate is best able to lead our nation as our president in the face of unprecedented challenges at home and abroad?" Clinton asked in an appeal aimed at superdelegates rather than Puerto Rican supporters who cheered her on but will get no vote in the general election.

Puerto Rico's role was enhanced in the Democratic primary election process this year as its 55 delegates are a critical chunk in the remaining three contests in a marathon primary season that has yet to crown a nominee. Pre-vote estimations had Clinton winning about 35 of the total delegates to Obama's possible 20.

Obama's aides are confident, however, they are only a stone's throw from the nomination. With 2,052 delegates heading into the day, Obama needed just 66 to get to the magic number of 2,118 delegates needed to claim the nomination. Clinton had 1,877 delegates going into the Puerto Rico contest. She picked up at least 38 delegates to his 16 in Puerto Rico. He also won the support of two superdelegates from other states.

The largely symbolic vote came with only two contests left on the calendar -- Montana and South Dakota, which both vote Tuesday.

Obama campaigned in South Dakota during the day, while Clinton was in Puerto Rico. He called her to congratulate her on the victory. Afterward, he held a rally in Mitchell, S.D., and pledged that despite the long primary season, the party will come together in November.

"I just got off the phone with Senator Clinton. She is going to win Puerto Rico and I wanted to congratulate her for that, and you know, I know there have been a lot of Democrats who have been concerned about whether or not the party is going to be able to come together after this long contest, and whether Clinton supporters will support Obama and whether Obama supporters will support Clinton's. Listen, first of all Senator Clinton is an outstanding public servant, she has worked tirelessly on this campaign, she has been a great senator for the state of New York and she is going to be a great asset when we go into November to make sure that we defeat the Republicans. That I can promise you," he said.

Sunday: Barack Obama speaks during a rally

at the Corn Palace in Mitchell, S.D. (AP Photo)

Lower-than-expected turnout in Puerto Rico could hurt Clinton's aim of arguing she can attract the crowds to the voting booth. Morning reports suggested turnout would end up in the 300,000 to 400,000 range rather than the 500,000 to 600,000 area that had originally been projected.

Using complicated math, Clinton is arguing to superdelegates that she has a higher popular vote total, but if turnout is lower than expected, that hurts her chance to pull away from Obama and use her argument that she can pull the big draw.

Kenneth McClintock, the Puerto Rico Senate president, said 20 percent turnout would be about 460,000 voters. That is roughly the number the Clinton campaign said they expected to see when all the votes are tallied.

McClintock said he was slightly disappointed by the lower turnout, which he attributed in part to Obama "throwing in the towel last night" and admitting he wasn't going to win. McClintock said that may have kept Clinton supporters home.

He added that the Democratic Party also did not put out as much information about the election as it has in the past and political parties were not in competition so it prevented that kind of mobilization.

Montana and South Dakota have a combined 31 delegates at stake on Tuesday, and mmathematically, Clinton faces a real challenge to getting the nomination -- needing about 96 percent of the remaining uncommitted superdelegates.

Since Super Tuesday, a majority of superdelegates have endorsed Obama and the trendline continues to be in his favor. Complicating Clinton's bid is an effort by House Speaker Nancy Pelosi, who has been neutral in the race, but says she wants uncommitted lawmakers to issue their endorsements soon after Tuesday.

Clinton is trying to keep her options open and the negotiating path clear, but has suggested that if she isn't offered the vice presidential nomination perhaps that could diminish her level of enthusiasm for the nominee.

Clinton's campaign has also said it is reserving the right to challenge the decision concerning Michigan's delegates. That decision on Saturday by the Democratic National Committee's Rules and Bylaws Committee gave Clinton 69 delegates to Obama's 59 even though he wasn't on the ballot in the state. Florida's entire delegation will be seated but each delegate will only get half a vote.

FOX News' Major Garrett and The Associated Press contributed to this report.