Mike Plante, who has run numerous campaigns for Democrats, said a conservative Democrat who supports the coal industry would have the best chance of winning the primary and beating Capito.
"They would have to carve out an identity that they're an independent-minded, West Virginia Democrat, not a Washington Democrat," said Plante. "They need to be pro-coal and conservative on social issues."
Democratic Party Chairman Larry Puccio said people "extremely interested" in running contacted him Friday. He declined to name them.
Puccio predicted that the U.S. Senate Democratic primary would be competitive, and unsuccessful candidates likely would support the winner.
"We showed that when the candidates supported Governor [Earl Ray] Tomblin last time," Puccio said Friday. "The winning candidate will be someone who people believe in. I think we'll be very successful in that election."
Carper said whoever runs must secure support from national as well as state Democratic Party leaders.
Candidates also must have tremendous personal wealth or be able to raise millions of dollars, political observers said.
"This will be the most expensive Senate race in the history of West Virginia," Carper said. "The race has national implications. At the end of the day, this is going to be about money."
The 2016 West Virginia gubernatorial race also could factor into the 2014 U.S. Senate race. Democratic candidates might run for Rockefeller's seat to make their names more recognizable with voters, then start a run for governor soon after.
"It certainly would be an opportunity to build some name recognition," said Chris Stadelman, vice president of The Manahan Group, a political consulting firm in West Virginia. "There's going to be a wide field -- not only for the Senate race in 2014 but for others, looking ahead to 2016.
Stadelman expects Democratic candidates to start announcing plans to run for the Senate in the coming weeks.
So far, Capito has no opposition in the GOP primary, but U.S. Rep. David McKinley has said he has not ruled out running against Capito. McKinley issued a statement praising Rockefeller Friday, but he did not say if he would run for the senator's open seat.
"There's going to be so much money involved in this race on both sides," Stadelman said, "so you have to announce fairly quickly so you can start raising money."
Carper said the winner of the Democratic primary would be someone who can raise money from national groups while also waging an aggressive grassroots campaign across West Virginia.
"They better have a fire in the belly," he said. "It has to be a tenacious campaigner."
And if there's consensus among Democrats it's this: Capito will be tough to beat. She's never lost an election.
"We were hoping that Senator Rockefeller would run again," said Bob Brown, executive director of the West Virginia School Service Personnel Association. "It's going to take a lot of money to beat Shelley."
Reach Eric Eyre at erice...@wvgazette.com or 304-348-4869.