Wall Street speculator Carl C. Icahn yesterday offered $8.5 billion in cash and securities for Phillips Petroleum Co., putting the nation's ninth-largest oil company under the threat of a takeover for the second time in two months.

But Icahn's bid is contingent upon his ability to borrow money to finance it -- something analysts were uncertain was possible -- and Phillips said it would not consider the offer until Icahn has lined up financial backing.

Some analysts, however, said that Icahn's bid could push Phillips into the arms of yet another suitor -- speculation centered on Atlantic Richfield Co. -- or force the company to buy its own stock at a price similar to Icahn's and go private under a leveraged buyout. A number of experts suggested that Icahn's offer was made just for that reason.

Phillips stock yesterday jumped $3.125 to $50.25 in active trading after Icahn's announcement. More than 5.1 million shares changed hands.

Icahn's $55-a-share bid comes as Phillips is trying to convince its shareholders to accept a controversial recapitalization plan, valued by the company at $53 a share but estimated by many on Wall Street to be worth less than that. The recapitalization plan, which is to go to a stockholders' vote later this month, is the key part of a package assembled by Phillips several weeks ago to chase off its other takeover threat, Mesa Petroleum Co. Chairman T. Boone Pickens Jr. Icahn has characterized the recapitalization plan as "grossly inadequate."

Wall Street analysts were divided on the effect of the offer by Icahn, a longtime speculator and financier who was one of several investors who bought big positions in Phillips in anticipation of a $60-a-share takeover by Pickens only to be left high and dry when Pickens and Phillips reached a truce and the price of Phillips stock slid into the mid-40s.

"I think that he's probably got [Phillips management] boxed in," said Alan Edgar, an oil-industry analyst at Schneider Bernet and Hickman in Dallas. "Fifty-five dollars is a lot higher and fairer than the blended price [of the recapitalization] in the high-40s . . . I think management, to try to force their transaction through with this price on the table, would find it backfiring."

Another analyst, Bruce Lazier at Prescott Ball & Turben, said Icahn's offer was another indication that Phillips would have trouble gaining stockholder approval of its recapitalization plan and might be better off looking at other alternatives, such as a leveraged buyout or a takeover by another major oil company. "I think that they've got to be thinking about a white knight at this point," he said. "I think that somebody's going to be invited into the party by Phillips."

Sanford Margoshes, who follows the oil industry for Shearson Lehman/American Express, said Icahn's offer puts Phillips more "in play" -- merger specialist parlance for companies vulnerable to takeover. "Maybe some other predator's interest has been piqued," he said.

But Margoshes said he did not believe Icahn's offer itself was a strong alternative to Phillips' recapitalization plan, because of the questions about its financing. "You have a known quantity at $53, and on the other side you have an unknown quantity at $55," he said. "I think that institutions and more conservative investors will opt for the known, even though on the surface the Icahn offer is $2 more." Nearly half of Phillips' 154.6 million shares are owned by institutional investors.

Icahn's proposal offers a 50-50 combination of cash and subordinated notes for Phillips shares. But Icahn said in a letter to Phillips that he had not yet arranged loans to pay for the cash part of the offer, although he said his financial adviser, Drexel Burnham Lambert, was confident it could put together the financing over the next couple of weeks if Phillips agreed to the offer.

Phillips, however, seemed unwilling even to consider the bid without the financing, thus apparently putting the two sides at an impasse. Neither Icahn nor Drexel Burnham officials could be reached for comment.

"There's no assurance that Mr. Icahn can come up with firm financing," Margoshes said. "My best guess is that tomorrow Phillips will graciously decline the offer."

Lazier said that while Icahn's and Phillips' positions "kind of put a Catch-22 in" for the time being, "the question is how long until somebody gets the financing."

Speculation has circulated on Wall Street for the past couple of weeks that Icahn or some other large shareholder in Phillips might make a bid for the company in an effort to recoup something from Mesa's abandonment of its earlier offer. Another name prominently mentioned has been Minneapolis investor Irwin L. Jacobs, who -- like Icahn -- owns something under 5 percent of Phillips stock. Jacobs could not be reached for comment yesterday.

The dissatisfied investors have been said to be massing opposition to Phillips' recapitalization plan, under which Phillips would purchase 38 percent of its own stock, greatly increase its debt and offer a roughly 42 percent interest in the company to Phillips employes. Phillips has argued that the plan is worth $53 a share to all of its shareholders, and thus would buoy its stock price, but Wall Street has disagreed. Phillips' price has stayed several dollars below the purported value of the recapitalization offer.

Phillips' shareholders are scheduled to vote on the recapitalization Feb. 22. Yesterday, Icahn asked Phillips to reschedule that meeting to allow shareholders to make a direct choice between his offer and the recapitalization plan.

Margoshes said that if the stockholders' meeting goes on as scheduled and the recapitalization plan is approved, "that makes all of today's events strictly academic . . . If, however, shareholders do not approve, then it makes today's offer a possibly viable offer."

Regardless of how Phillips shareholders vote, Phillips has promised Pickens $53 a share for Mesa's holding in the company. Yesterday, Pickens said he was evaluating Icahn's offer, but he added, "We really are committed to Phillips under the agreement." However, Pickens noted that those eligible to vote at the shareholders meeting would be holders of stock as of last Friday, so that in theory, Mesa could sell its shares to Icahn or another buyer but still cast their votes for the recapitalization plan.