The highest-profile Republican supporter of human embryonic stem cell research in the US House of Representatives has fallen to a virtual unknown in his quest for a US Senate seat. Mike Castle, a longtime congressman and former governor of Delaware who was the Republican establishment’s candidate, was beaten handily on 14 September by Christine O’Donnell, a conservative abortion opponent with strong backing from the Tea Party movement. The two were vying for the right to represent the Republican Party in a closely-watched race for the US Senate seat vacated last year by Vice-President Joe Biden.

With 99 percent of precincts in the state having reported results late on Tuesday evening, O Donnell, a marketing consultant, led with 53.1 percent of the vote to Castle’s 46.9 percent.

“The voters in the Republican primary have spoken and I respect that decision,” Castle told supporters in a concession speech. “I would like to thank the Republican Party for its support.”

While economic issues played a paramount role in the hotly contested election, O’Donnell had also attacked Castle for his outspoken support of government funding for stem cell research, which is controversial because human embryos must be destroyed to derive stem cells.

O’Donnell will represent the Republicans in the general election in November against Democrat Chris Coons, a county-level politician who was unopposed for the Democratic nomination. O’Donnell’s win will make the contest a tougher one for Republicans when the general electorate comes to the polls to decide who will serve out the remainder of Biden’s term, which ends in 2014. (Only Republicans may vote in Delaware’s closed primary.)

Castle is the leading Republican sponsor of a bill passed by Congress twice during the George W. Bush administration, and twice vetoed by Bush. The Stem Cell Research Advancement Act would make government funding for human embryonic stem cell research explicitly legal, provided that cell lines are derived with private funds from leftover embryos at fertility clinics that would otherwise be destroyed. The bill also stipulates that couples must not have been offered inducements to donate, and must have given written informed consent.

Questioned after a U.S. district court judge issued a preliminary injunction halting government funding for the research on 23 August, Castle said that he still supports the bill, which was reintroduced in March this year by Castle’s Democratic cosponsor, Diana DeGette (Colorado.) The injunction was temporarily reversed last week by the US Court of Appeals for the District of Columbia Circuit.

Proponents of the research are pushing Congress to act quickly to make government funding for the research explicitly legal.

“I still have some time in office. I will still be fervently trying to carry out my responsibilities,” Castle said of his remaining term in the House of Representatives, which ends in early January.