"If I am so fortunate and honored to have the support of the people of West Virginia, I can't fill his shoes," Manchin said. "I only hope I would be able to follow in his footsteps and continue to help people of West Virginia."
Manchin's decision came after he and legislative leaders resolved their differences over the succession process. The legislation calls for an Aug. 28 primary and Nov. 2 general election to fill the roughly two years that would be left in Byrd's term. It also sets a four-day candidate filing period, which started Tuesday.
The Nov. 2 winner would take over that month from Carte Goodwin, 36, Manchin's temporary appointee. The governor's former chief counsel took his oath of office Tuesday and has said he will not run for the seat.
Republicans view their top prospect as Rep. Shelley Moore Capito, who serves in the state's 2nd Congressional District. State GOP lawmakers won an amendment in the legislation that would allow her to seek Byrd's seat without abandoning her ongoing bid for a sixth U.S. House term. They had also complained the measure benefited Manchin and deterred serious challengers.
The November election will be key for Democrats trying to hold onto a slim majority in Congress. Were Manchin to lose, he would remain governor.
Manchin has won three consecutive statewide races, and he would have the edge if Capito entered the race, said West Virginia Weslyan College political science professor Robert Rupp.
"It would be a short race by qualified, experienced, effective campaigners," Rupp said. "They're already well-liked in the state."
GOP attacks on Manchin began before his formal candidacy. The National Republican Senatorial Committee has filed Freedom of Information Act requests with his office, seeking fodder for attacks and the National Republican Committee cast Manchin as a would-be rubber stamp for President Barack Obama with a liberal record.
Obama lost West Virginia handily to GOP nominee John McCain in 2008 and is considered unpopular here. But Manchin carried all 55 counties in that election to land his second term as governor, winning nearly 70 percent of the vote. Registered Democrats have a nearly two-to-one edge over Republicans in the state.
As governor, Manchin has had mixed views on both the 2009 federal stimulus program and the sweeping health care overhaul passed earlier this year. He has repeatedly sparred with Democrats in the White House and Congress over efforts to cap greenhouse gas emissions caused by burning coal.
Manchin has instead urged a more gradual approach that provides time and funding to develop cleaner methods for using coal and, eventually, alternative fuels. He has frequently criticized Obama administration regulators as well over their handling of mining permits.
"This is a governor who has taken his time to distance himself from the Obama administration," Rupp said, dismissing the initial national GOP attacks as tone-deaf.
Manchin pushed for stronger mine safety and rescue measures after the Sago disaster. But in that tragedy, he also relayed a rumor to relatives that the miners had survived. That proved false, leaving him to tell the devastated families later that all but one of the dozen trapped miners had died.
Manchin's conservative fiscal approach to state finances recently won the state an improved bond credit ratings from Moody's Investors Service. Such groups as the Cato Institute have applauded Manchin's push for gradual tax cuts benefiting both businesses and consumers. During his tenure, West Virginia has avoided the recession-induced tax hikes, public worker layoffs and reduced government services suffered in many other states.
Manchin said he would pursue such policies while championing the state's role in the nation's energy needs with coal.
"Now I get to tell the whole world, maybe, how good you are," Manchin said.