FACTBOX: Why does Hillary Clinton keep running?

(Reuters) - Hillary Clinton has shrugged off calls to drop out of the 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 the final six contests have been concluded on June 3.

* Given that she is favored in three of the upcoming contests -- West Virginia, Kentucky and Puerto Rico -- she may want to continue until all the contests are concluded in order to feel like she has exhausted her options and explored every last possible avenue.

* 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 to be the stronger candidate. That would let her 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 ended March in heavy debt and has since loaned $6.4 million from her own pocket to fund her bid, could need more time to raise money, pay off her debts and repay her loans.

* She and her husband, former President Bill Clinton, could be interested in protecting their legacy and ensuring the duel ends on a graceful note that would repair her image and revive their fading reputation in the black community, which has backed Obama heavily.

* She could be angling for a leadership role in the U.S. Senate.