Governor Joe Manchin chose his former chief counsel, 36-year-old Mr Goodwin, to fill the seat previously occupied by Mr Byrd, the US’s longest serving senator, who died last month at the age of 92.

Mr Goodwin will be sworn in on Tuesday and will serve until November.

Mr Manchin, a Democrat, is tipped to run for the seat when elections are held. State lawmakers are discussing when a special election will be held, before the seat is filled for the full six-year term during November’s nationwide Congressional elections.

Mr Goodwin worked as Mr Manchin’s lead attorney between 2005 and last year, when he joined his family’s law firm. He is not expected to run in the special election, local media reported on Friday.

Democratic leaders in Congress are eager to have the seat filled quickly because without Mr Byrd’s vote Democrats have been two votes short of the super-majority they need to ram through legislation.

Even though they will still be one vote short after Mr Goodwin takes the seat, his vote will mean Democrats will need to win over one less Republican to pass legislation. This week’s financial regulatory reform bill was passed with the support of three Republicans.

Mr Goodwin’s vote is not, however, likely to make the passage of climate change legislation any easier. Democrats plan this month to put forward a bill that would promote clean energy and crack down on fossil fuels, a proposition that is not popular in West Virginia, a coal mining state.

The uncertainty over Bakiyev’s position has disrupted flights out of the Kyrgyz air base which the United States rents to support the war in Afghanistan, and Russian President Dmitry Medvedev warned that the impoverished Central Asian country may be on the brink of civil war.

“I feel optimistic about the steps (the interim government) is already taking … the United States is prepared to help,” Robert Blake, US assistant secretary of State, told reporters after meeting interim government chief Roza Otunbayeva.

Blake is the highest profile US official to visit Kyrgyzstan since Bakiyev’s opponents took power after an April 7 uprising during which troops repeatedly fired into crowds of opposition protesters in the capital. At least 84 people died in the clashes.

Ms Otunbayeva said the Manas air base was not discussed and that Mr Bakiyev must be held accountable for the protest deaths.

“He must stand trial,” said Ms Otunbayeva. “If we get our hands on Bakiyev, then he will be put on trial. He has already had his chance to leave,” she told reporters after meeting Mr Blake.

Mr Bakiyev denies giving the order to fire into the crowds, but his brother Dzhanibek Bakiyev, the chief of the presidential bodyguard, has admitted doing so.

Washington is concerned the crisis will affect its five-year lease deal with Kyrgyzstan for use of Manas. After last week’s upheaval, members of Ms Otunbayeva’s interim government had suggested the lease would be shortened.

Russia also has an air base in the country and Moscow has long sought to evict the US from Central Asia, a strategically important region wedged between China, Afghanistan and the Caspian Sea.

The interim government says it will abide by its agreements on Manas, but its pro-Russian sympathies have given rise to suspicions the Kremlin may try to use the base as a lever in its relations with Washington.

Mr Medvedev suggested Bakiyev should formally step down to defuse a crisis he said could develop into a “second Afghanistan”.

“As I understand it, Kyrgyzstan is on the verge of civil war,” Mr Medvedev told an audience at a think-tank in Washington, where he was attending the global nuclear security summit.

After the provisional government threatened to send special forces to arrest him, Mr Bakiyev on Wednesday hinted that he might leave the country if the government guaranteed his safety and that of his family.

“I am not clutching at my armchair and I have not said that I am not going to step down under any circumstances,” he told reporters in his village.

“What I said is, that if the issues of my personal safety and the safety of my family members are resolved … and if there is stability in Kyrgyzstan, then I am ready to consider this question,” he said.

“To argue that the president of Kyrgyzstan would not under any circumstances step down and that he would not leave the country is not the way the question should be posed,” he said.

Mr Bakiyev’s sharp change of tone – after days of defiance and veiled threats of conflict – could open up a path out of the turmoil, though fears of ethnic strife reared their head on Wednesday when Uzbeks rallied in the southern city of Jalalabad.

“I feel something is being cooked up. I feel that something bad is in the air,” said Mr Muradillo, a 34-year-old Uzbek musician in Jalalabad, who gave only his first name for safety reasons.

Jalalabad is in the Ferghana valley, a cauldron of ethnic and tribal tension in the heart of Central Asia that was the scene of violent clashes in the last days of the Soviet Union.

Uzbeks in the region demanded autonomy from the Soviet republic of Kyrgyzstan, provoking a deadly backlash that killed at least 300 people.

At least 2,000 ethnic Uzbeks attended a rally on the central square in Jalalabad on Wednesday to protest against Mr Bakiyev. Later, about 1,000 Kyrgyz Bakiyev supporters rallied at the same square.