WASHINGTON, D.C. -- Sen. Claire McCaskill, D-Missouri, is hardly a household name outside her home state.  Sen. Barack Obama, D-Illinois, knows who she is, however.

McCaskill was there when Obama needed her most, a female senator endorsing him just after he lost New Hampshire's Democratic presidential primary to Hillary Rodham Clinton. That was a politically risky move at the time, one that angered many of her supporters. It came when it was far from certain that Obama could win the party's nomination.

Since then, the plainspoken former prosecutor and state auditor has been all over TV news and political talk shows as a top surrogate for the campaign. Obama calls her one of his closest advisers. She's even offering guidance on possible vice presidential picks and her name has popped up as a potential running mate.

''They want to use her so much because she's the epitome of the target voter they're looking for,'' said Democratic strategist Jenny Backus.

For Obama, McCaskill has been crucial in reaching out to some of the female voters who flocked to Clinton during the primaries.

If Obama's rapid rise in national politics has been remarkable, so has McCaskill's. Three years ago, she thought her political career might be over after she lost a bid for Missouri governor.

Then she ran for the Senate -- and won.  Political advisers see in her victory a blueprint for how Obama, too, can carry evenly divided swing states in the general election in November against Republican Sen. John McCain.

Anita Dunn, a senior adviser to Obama's campaign, said there is no chance that Obama could have eked out a narrow win in the Missouri presidential primary without McCaskill's strategic help. The campaign is tapping that insight for other states in the general election.

''Her insights into campaigning in the more rural areas are ones she has shared directly with Barack,'' Dunn said. ''Her basic thing is that you've got to get down there and meet with people. You've got to let them see you.''

As it became clearer that Obama would claim his party's nomination, McCaskill was among those urging him to make stops in places like Cape Girardeau, Mo., and southwest Virginia -- conservative country where Democrats seldom tread.

''I will make a bold statement,'' McCaskill said in an interview. ''Barack Obama will campaign in places that no presidential candidate has ever campaigned in, much less a Democrat.''

The need for Democrats to campaign hard in rural and conservative areas is a lesson McCaskill learned the hard way.  After losing the 2004 gubernatorial race to Republican Matt Blunt, she said her biggest mistake was ignoring Republican Party bastions where she got trounced.

Two years later, McCaskill narrowly defeated incumbent Republican Sen. Jim Talent. She racked up huge margins in the Democratic Party strongholds of St. Louis and Kansas City but it was her ability to cut into Talent's rural support that put her over the top.  McCaskill won those votes by spending weeks in an RV touring counties where Democrats are scarce.

''John Kerry never landed an airplane outside of Kansas City or St. Louis,'' McCaskill said of her party's 2004 presidential nominee.

Kerry's campaign ultimately abandoned Missouri in the waning days of the race and lost the state.

Focusing on the big cities sends the wrong signal, McCaskill said.

''It sends a signal that this is about turning out Democratic voters rather than persuading everybody in America that you can be their champion," she said.

McCaskill's advice has made a difference in more subtle ways, too. She said Obama sometimes has ''a tendency to explain the intellectual rationale for something he's done before he explains what was going on in his heart.''

After his remarks in Pennsylvania that rural voters bitter over the economy ''cling to guns and religion,'' Obama tried to explain by saying his ''syntax was poor.''

''I teased him about using the word syntax,'' McCaskill said. ''I said, 'Barack, where I come from that's the tax you have to pay on beer.' Sometimes it's better to say 'I screwed up' than to say 'I got the wrong syntax.'''

Observers say McCaskill's ability to come across in simple terms is what has made her so appealing as a surrogate.

''She's an older, middle-aged, woman who speaks clearly in a language people can understand,'' said Democratic strategist Dane Strother. ''She fits the demographics and the geographics Obama needs, which is basically Hillary's base.''

Asked how much the campaign intends to use McCaskill, 54, in the general election, Dunn said: ''As much as possible. She'll obviously play a huge role in Missouri and continue to be one of our top television surrogates.''

''You can't underestimate the importance of Claire McCaskill to this campaign,'' Dunn said.