Updated

Republicans pointed to Delaware Attorney General Beau Biden's decision not to seek his father's former U.S. Senate seat Monday as a "major recruiting setback" for Democrats which only adds to the mounting challenges they face in defending their Senate majority.

Beau Biden announced he would not run after Vice President Biden appeared to meddle in the race -- though the vice president's office says he wasn't trying to push his son's candidacy for his old seat but was actually pumping up current Sen. Ted Kaufman, Biden's former chief of staff.

Kaufman said Monday he doesn't want six more years in Congress.

Harry F. Themal, a columnist for the Wilmington, Del., News Journal, wrote in his Sunday column that the vice president urged him to convince his son to run for Senate.

"If you run into Beau, talk him into running; he respects you," Biden reportedly told the writer.

More On This...

But Politico.com reported that according to a transcript of the conversation provided by Biden's office, Biden was referring to placeholder Kaufman.

"Talk Ted into running, if Beau doesn't," Biden reportedly said. "Talk him into running -- he respects you."

Either way, the attempt failed. Beau Biden said Monday he had given "serious consideration" to running for Senate but ultimately decided to run for re-election as attorney general.

"My first responsibilities are here in Delaware," he said. "Therefore, I cannot and will not run for the United States Senate in 2010."

Kaufman followed with a statement soon after, affirming that he would not seek election for the remainder of the term.

That leaves Republican Rep. Mike Castle, a former Delaware governor and the state's sole congressman, in the race without Democratic opposition.

Castle said in a statement Monday that he respects Biden's decision, and that it is important for the state to have an "independent voice" in the Senate.

"While it remains uncertain who my opponent will be, my commitment to representing Delawareans has never been stronger," he said. "In November, I hope to have earned the trust of Delaware voters and that I will have the honor of representing their interests in the United States Senate."

National Republican Senatorial Committee spokesman Colin Reed said Castle is now in a "strong position" to win back the seat.

"Clearly, Beau Biden's decision not to run for the United States Senate represents a major recruiting setback for national Democrats, who have been counting on his candidacy to keep the vice president's former seat in Democrat control," Reed said. "As we saw in Massachusetts last week, voters clearly stated that these seats belong to the people - not to either political party or dynasty."

Democrats on Tuesday saw the seat formerly held by the late Sen. Ted Kennedy in Massachusetts fall to Republican Scott Brown. The win raised questions about Democrats' ability to hold onto seats once thought to be reliable for their party.

Delaware has been a solidly Democratic state, according to recent polling years, with Joe Biden being simultaneously elected vice president and re-elected senator in 2008. Democrats John Kerry and Al Gore convincingly defeating George W. Bush in 2004 and 2000, respectively. But Castle is serving his ninth term and won with 61 percent of the vote in 2008.