FACTBOX: Hillary Clinton's last stand in Ohio and Texas

(Reuters) - Democrat Hillary Clinton faces two crucial and potentially "must-win" showdowns with presidential rival Barack Obama in Ohio and Texas on Tuesday.

Here are some views on Clinton's challenges and her options if she fails to win both states with big enough margins to cut into Obama's current lead in pledged delegates, who will choose the Democratic nominee in November's election.

BARACK OBAMA, Illinois senator and candidate, in ABC interview.

"If we do well in Texas and Ohio, I think the math is such where it's going to be hard for her to win the nomination. And they'll have to make a decision about how much longer they want to pursue it."

CAL JILLSON, a political scientist at Southern Methodist University in Dallas.

"There will be people in her campaign who will argue a split decision is good enough to continue. But as a practical matter, just looking at the delegates, the numbers won't really add up for her."

BILL RICHARDSON, governor of New Mexico who dropped his presidential bid in January, on CBS' "Face the Nation."

"We have to have a positive campaign after Tuesday. Whoever has the most delegates after Tuesday, a clear lead, should be, in my judgment, the nominee."

MARK PENN, Clinton strategist.

"This has been a roller-coaster race, and every time people try to foreclose it the voters come back and say, 'Just wait a minute here; we want to have our say.' Let's see what the voters say, let's take a look at what the situation looks like on Wednesday, and I think it will be pretty clear."

HOWARD WOLFSON, Clinton spokesman

"We are very optimistic about our chance of success in Ohio and Texas. If the outcome is different we can talk about it then."

FORMER PRESIDENT BILL CLINTON, Clinton's husband, speaking at a Texas rally

"If she wins Texas and Ohio I think she will be the nominee. If you don't deliver for her, I don't think she can be. It's all on you."

(Writing by John Whitesides; Editing by Cynthia Osterman)

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