The last time Angus King ran for office he easily surpassed his four competitors’ fundraising efforts in an inexpensive race.

The figures in King’s 1998 gubernatorial re-election bid are likely to seem paltry in comparison with what will be spent on the contest for the U.S. Senate seat he’s now pursuing.

In 1998, King, an independent, spent about $785,000 — nearly four times as much the other candidates’ combined expenditures. King accomplished that even after he imposed a $250 cap on contributions for his campaign, a limit to which the vast majority of his donors adhered. At the time the legal limit was $1,000 from individuals and $5,000 from corporations.

It’s not clear how much will be spent to win the seat being vacated by U.S. Sen. Olympia Snowe — or even who King will be competing against. It’s a race that involves an unexpectedly open seat and one that is likely to draw national attention at a time when outside groups can spend unlimited amounts in support of a candidate.

A “modest” campaign budget could require $2 million to $3 million — and the figure could rise sharply depending on who else is in the race, according to one national Democratic consultant.

King acknowledged that campaigns are more expensive than when he last ran for office, when he won with about 59 percent of the vote. He will be skipping out on one costly expense — negative television advertising, which he believes is ineffective in Maine anyway — but said he may have to run ads to defend himself.

“Money is important in politics, but I don’t think it’s the whole deal by any means,” King said Friday.

The minimum amount needed for King’s campaign will depend on the other candidates and the money put into the race by the national parties and others from outside of Maine, said Jamie Broder, King’s finance co-chair. He said the King campaign will not be able to compete on a dollar-to-dollar basis with national party funding but does not need to because King is already well-known throughout Maine.

“It’s a question of communicating to the people of Maine his ideas and how he can serve in Washington in a situation where the Congress is a broken institution and can’t seem do to the people’s business. It’s not going to take $10 million to convey that message,” said Broder, who was King’s finance chair in 1994 and 1998 and co-chair of his transition team in 1994.

In 1998, contributions to King’s campaign totaled $775,876.74. He raised $371,323.81 from individuals and $81,971.83 from corporations. King and his wife contributed $10,479,58 and loaned the campaign $289,140.

The individual contributors include a good number of lawyers, educators, fellow Brunswick residents and chiropractors — who were grateful to King for signing a bill that allowed patients to see them without a referral from a primary care physician, according to John Royce, executive director of the Maine Chiropractic Association.

There are also retirees, homemakers, the self-employed and representatives of the non-profit sector.

Bath Iron Works employees are well represented. The late Duane “Buzz” Fitzgerald, the company’s former president, was among the core King supporters. There are other well-known names from business like Leon Gorham of L.L. Bean, Thomas Chappell of Tom’s of Maine, David Shaw of Idexx and David DeLorme of DeLorme.

It can’t be assumed that King’s 1998 donors will contribute to his Senate race, said Douglas Hodgkin, professor emeritus of political science at Bates College and a Republican activist.

For one, King no longer has the advantage of being a popular incumbent, Hodgkin said. The Senate race is taking place when the parties are more ideological, “so they’re not going to roll over and play dead,” he said.

Also a factor, Hodgkin said, is the greater availability of national money because of a 2010 Supreme Court ruling that allows political action groups to accept unlimited contributions.

“It’s a question of who King will get money from on the national scene. There could be some individuals out there that would be able to fund his campaign, but he doesn’t have a ready-made party organization and set of interest groups as do the two parties,” he said.

Ronald Schmidt Jr., a University of Southern Maine political scientist, said some corporate donors may believe it makes good business sense to continue their support of King — and contribute to the other candidates as well. But an independent may seem like a riskier prospect because there will be a greater question about how much he can do for their industries, he said.

“A big question if you’re thinking about strategic contributions is: unlike a governor, senators specialize. If he’s an independent it may be harder for him to get the committees you want,” Schmidt said.

National Democratic consultant Peter Fenn said how much King will need to raise to wage a competitive race will depend on how serious a Democrat is on the ballot.

If a serious Democratic candidate on the order of former Gov. John Baldacci is not on the November ballot, King could effectively be involved in just a two-way race against the GOP nominee, Fenn said.

That might mean a “modest” campaign budget of $2 million to $3 million is required. “But if it is a serious three-way race that number doubles or triples probably,” Fenn said.

Nathan Gonzales, deputy editor of the nonpartisan Rothenberg Political Report in Washington, agreed that King’s fundraising needs can’t be assessed for sure until general election opponents are decided.

“But it’s tough to see a new senator getting elected without at least a couple million dollars,” Gonzales said.

And that’s just what the candidates themselves will need to raise, said Jennifer Duffy, senior editor at the nonpartisan Cook Political Report in Washington. Super PACs will likely become major factors in a race that could decide the balance of power in the Senate, she said.

Democratic donors will find ways to support King, predicted Larry Sabato, director of the Center for Politics at the University of Virginia.

Sabato and Duffy were referring to the fact that many analysts believe — and some Democrats in Maine and nationally have acknowledged — that Rep. Chellie Pingree’s exit Wednesday as a prospective Senate candidate could lead Democrats to at least tacitly support the candidacy of King, a social liberal who has contributed money to President Obama’s campaigns.

“King won’t be married to the Democrats. It will be more like cohabitation, with the finances pooled,” Sabato said. “The Snowe retirement has given the Democrats a tremendous psychological boost nationally, and they are not about to blow this one.”

Sabato said he doesn’t know how much money King will need, but added that “I’ve yet to see a competitive Senate race that didn’t cost millions, even in a small state, in recent times.”

MaineToday Media Washington Bureau Chief Jonathan Riskind contributed to this report.