There's been a lot of talk about what Democrats will do if they lose seats in the House and Senate this November. Many Republicans believe Nancy Pelosi and Harry Reid will make a last-ditch, lame-duck effort to pass the remaining parts of their agenda, like tax increases, card check, and a climate change bill. And that's why Christine O'Donnell says the Senate race in Delaware is so important.

The Delaware contest is not a regularly scheduled election but rather a special vote to fill the last four years of Vice President Biden's old term. That means the winner will be sworn in immediately after the Nov. 2 election -- not in January like other new lawmakers -- and the new senator will take part in lame-duck votes.

O'Donnell is running for the Republican nomination against GOP moderate Rep. Mike Castle. If Castle wins the Sept. 14 party primary, she argues, he could go on to win the general election and start voting with Democrats right away.

"Republicans have to decide, who do they want serving in that lame duck session?" O'Donnell asks. "Do they want Mike Castle, who voted for cap and trade, voted for TARP, voted for cash for clunkers, or do they want a true constitutionalist?"

There's no doubt Castle is among the most liberal Republicans on Capitol Hill. In 2008, he received a rating of 28 (out of 100) from the American Conservative Union, while receiving a rating of 65 from the liberal Americans for Democratic Action. Most recently, he co-authored the DISCLOSE Act, which nearly all Republicans opposed.

O'Donnell is running on a solidly conservative platform, much like she did in 2008, when she challenged Biden and won just 35 percent of the vote. Now, her biggest job is convincing people that this race -- a primary open only to Republicans -- is one she can win.

For months, Washington-based Republicans have counted Delaware as a GOP pickup, but they've just assumed the Republican candidate would be Castle. The 71-year-old congressman is a former governor who has been in the House since 1993. He once led Democratic candidate Chris Coons by a huge margin, but that's changing. A recent Rasmussen poll found Castle with a 47 percent to 36 percent lead over Coons, while in the same survey, O'Donnell led Coons by 41 percent to 39 percent.

The state Republican Party is in Castle's corner; Chairman Tom Ross is openly critical of O'Donnell. But others in the GOP are lining up for her. The Tea Party Express and Concerned Women for America have endorsed O'Donnell, and she's hoping to get a nod from Republican Sen. Jim DeMint and even Sarah Palin. "In terms of her issues and where she stands, it's night and day compared to Castle," says Matt Hoskins, spokesman for DeMint's PAC. "Sen. DeMint recently met with her and was impressed."

O'Donnell's weak spot appears to be a checkered financial history. She attended Fairleigh-Dickinson University but was not allowed to graduate in 1993 because of unpaid tuition. (She actually went to the ceremony in cap and gown, she recalls, and "I was handed a leather portfolio, and inside, instead of a diploma, there was a bursar's bill.") It took her 10 years to pay it off.

Last year O'Donnell was audited by the Internal Revenue Service. In March of this year, the feds filed a lien for $11,744 in unpaid taxes. O'Donnell, a free-lance marketing consultant, says it was a mistake. "The IRS admitted it was wrong," she says. "They've zeroed out the balance and acknowledged that it was a computer error."

And then there's her house. There have been reports it was in foreclosure when she sold it during her 2008 run against Biden. That's a "flat-out lie," she says, although she did fall behind on payments.

Now, at 40, O'Donnell says she is in the clear. "I paid off all my personal debt," she says. "I paid off all my student loans, and I'm even moving to pay off my campaign debt."

The Castle camp has been quietly pushing the O'Donnell debt angle. But it seems less eager to talk about Castle's own agenda; a campaign spokeswoman did not respond to three requests for comment.

The Delaware primary is an example of the fight going on inside the GOP. A great opportunity is approaching. There's an older, established, and, in the view of many, squishy candidate running against a younger upstart who has flaws the opposition can exploit.

In the past, the upstart wouldn't have had a chance. But this year, no seat, and no established candidate, is truly safe.

Byron York, The Examiner's chief political correspondent, can be contacted at byork@washingtonexaminer.com. His column appears on Tuesday and Friday, and his stories and blog posts appears on www.ExaminerPolitics.com ExaminerPolitics.com