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 a nearly insurmountable lead in their fight for the right to face Republican John McCain in November's presidential election, and is about 45 delegates short of clinching the nomination.

Here are some possible reasons why Clinton, a New York senator and former first lady, is staying in the race, at least through Tuesday when the last two contests are held in Montana and South Dakota.

* She may want to continue so she feels like she has exhausted her options and explored every last possible avenue.

* She could be hoping wins in the final contests will help bolster her claim to an edge in the popular vote. 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.

* Clinton, whose campaign is more than $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.

* She and her husband, former President Bill Clinton, could be interested in protecting their legacy and ensuring the race 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.

(Writing by John Whitesides; editing by Mohammad Zargham and Bill Trott)

(To read more about the U.S. political campaign, visit Reuters "Tales from the Trail: 2008" online at http:/blogs.reuters.com/trail08/)