In the nearly 12 months since Vice President Joseph Biden moved to the other end of Pennsylvania Avenue, it has been a foregone conclusion in national and Delaware political circles that his eldest son, state Attorney General Beau Biden (D), would run in the special election in November for his father’s former Senate seat.

Appointed Sen. Ted Kaufman (D), a longtime friend and political aide to Joseph Biden, was viewed as a seat-warmer for Beau Biden, and officials in both parties have been gearing up for a generational showdown between the 40-year-old Biden and 70-year-old Rep. Mike Castle, the likely Republican nominee.

But what happens if Biden doesn’t run?

Few have asked the question because of the perceived inevitability of a Biden Senate campaign. But at this stage, it appears that Democrats don’t have a Plan B — it’s Biden or bust.

Delaware Democrats have been deferring to Biden as he nears a final decision. He’s expected to make an announcement sometime next month, after the holiday season.

“We still fully hope and expect Beau Biden to run,— said Eric Schultz, a spokesman for the Democratic Senatorial Campaign Committee. “It’s critical that we hold on to the vice president’s seat, so we’re going to make sure that state has the resources that it needs in order to win.—

John Daniello, chairman of the Delaware Democratic Party, said he speaks to Beau Biden once or twice per month but declined comment when asked if he thought Biden would run for the Senate.

Daniello said there is plenty of time to wage a Senate campaign.

“This urgency for people to announce before we’re even into the year of the election doesn’t make sense to me,— Daniello said.

If Biden does decide to run, the Delaware race would become among the most competitive of the 2010 cycle. But in his absence, Democrats would have to scramble to field a serious candidate and prevent Republicans from winning the seat for the first time since 1960.

Though the state has been trending Democratic, the party doesn’t have a replacement candidate of Biden’s political stature. Former Lt. Gov. John Carney, who was passed over for the Senate appointment that went to Kaufman, is running to replace Castle in the House and wouldn’t challenge Castle for the Senate seat. Chris Coons, the New Castle County executive, might be interested in running if Biden doesn’t, though he’s not well-known statewide.

Kaufman has said he won’t run in 2010, though there’s been some speculation that the Biden family might try to persuade Kaufman to reconsider if Beau Biden chooses not to run.

Biden, meanwhile, has avoided much public comment about his political future since returning in late September from a year in Iraq as a captain in the Delaware Army National Guard.

“Am I considering it? Absolutely. Absolutely,— Biden said in mid-October on “Good Morning America.— “But I’ll be making a decision in due course.—

Political analysts cautioned against interpreting Biden’s reticence as a sign that he may not run. Delaware is a sparsely populated state, and Biden has the near-universal name recognition that other prospective candidates lack. Biden’s familial tie to the White House would enable him to raise the funds needed to wage a top-flight campaign.

And by delaying an announcement until January, Biden would not have to file a campaign finance report until mid-April, keeping political opponents in the dark about the strength of his fundraising.

Even as Biden has been mum about the Senate race, his office has been actively promoting its accomplishments — including beefing up penalties for child predators and a lawsuit Delaware is pursuing to block the Army Corps of Engineers from deepening the Delaware River.

There are political risks for Biden in a Senate campaign. Not least is that the Republicans have fielded their strongest possible opponent in Castle, the state’s at-large Representative since 1993 and a former governor who has crossover appeal to independents and Democrats. Castle led Biden 45 percent to 39 percent in a poll conducted Nov. 30-Dec. 2 by the Democratic firm Public Policy Polling Inc.

And at just 40 years old, Biden will have other opportunities to run for Senate. If Castle is elected next year, he would be up for re-election in 2014, when he will turn 75. The state’s other Senator, Tom Carper (D), will be 65 when his seat comes up next in 2012.

Democratic strategists think they could win a Castle-Biden race in part by running a campaign on a theme of generational change between Biden and Castle. They think it could parallel the state’s 2000 Senate race, in which the 53-year-old Carper unseated 79-year-old Sen. Bill Roth (R) even though Roth was a well-regarded veteran officeholder.

With no candidate to promote just yet, Democratic officials are training their fire on Castle in an effort to erode his standing in the state. They claim his voting record has moved to the right.

After Castle voted last week against a Democratic-written overhaul of financial regulations, the DSCC said Castle “gave a big wet kiss to Wall Street.—

After the House last month narrowly passed a bill to overhaul health care policy, Democratic strategists speculated that Castle’s “no— vote would be a political liability in the Senate race. But the PPP survey found that voters, by 46 percent to 43 percent, oppose rather than support the health care measure.

Castle is “an independent, experienced statesman, and he’s earned respect from both Republicans and Democrats,— said Brian Walsh, a spokesman for the National Republican Senatorial Committee. “Whether it’s Beau Biden or anyone else, they’re going to have a very uphill climb against someone like Mike Castle.—