FACTBOX: Why does Hillary Clinton keep running?

(Reuters) - Hillary Clinton has shrugged off calls to drop out of the U.S. Democratic presidential race and said she will keep running "until there is a nominee."

But rival Barack Obama has opened an insurmountable lead in pledged delegates in their fight for the right to face Republican John McCain in November's presidential election.

Here are some possible reasons why Clinton, a New York senator and former first lady, is staying in the race until June 3 to finish the remaining three contests -- in Puerto Rico, Montana and South Dakota.

* Given that she is favored in Puerto Rico, she may want to continue so she feels like she has exhausted her options and explored every last possible avenue.

* Results from two big states -- Michigan and Florida -- were disqualified in a dispute with the national party. Clinton won both, although there was no campaigning and Obama was not even on the ballot in Michigan. Clinton may hope a decision to seat delegations from Florida and Michigan at the party's August convention would pull her close enough to Obama to sway superdelegates in her favor.

* She could be hoping for big wins in the remaining contests that might let her claim an edge in the popular vote when Michigan and Florida are included. While national popular vote plays no role in the selection process, a Clinton lead could sway superdelegates to her side.

* She could be staying in the race to see if Obama makes a gaffe or becomes embroiled in another controversy that would rekindle voter concerns about his inexperience and make her appear the stronger candidate. She could then make the case to superdelegates that she has the best chance to beat McCain.

* She could be trying to pressure Obama to add her to the ticket as his vice presidential nominee. That idea was floated by a former Clinton aide, ABC News television personality George Stephanopoulos.

* Clinton, whose campaign is at least $20 million in debt, may need more time to raise money and pay off her campaign debts, including the more than $11 million she has given the campaign from her own pocket.