The bidding for Conoco Inc. reached a frenzied pitch today as the most expensive corporate takeover battle came down to the wire.

Wall Street experts said that E.I. duPont de Nemours & Co. appears certain to win the three-way battle with Mobil Corp. -- whose bid is under antitrust scruntiny by the Justice Department -- and Seagram Co., which last Saturday began paying stockholders who sent it shares.

As of Monday the chemical giant had more than 49 million shares of Conoco stock on hand, enough to give it control, though shareholders have until midnight tonight to withdraw them from Du Pont. Du Pont plans to start paying for those shares shortly after midnight. Conoco shareholders can continue to send Du Pont stock until Aug. 17.

In an attempt to cement its apparently winning offer. Du Pont today boosted from $95 to $98 a share its cash offer for 38.7 million, or 45 percent, of outstanding Conoco stock. Du Pont will continue to exchange 1.7 shares of its stock for each of the remaining 48 million shares. Du Pont's offer now is worth more than $7.4 billion.

Mobil, trying to convince Conoco shareholders to withdraw stock from Du Pont while they can, responded to the Du Pont move by adding $5 a share to the cash portion of its offer, already the richest of the three. Mobil said it will pay $120 a share for 43.5 million shares and continue to exchange Mobil securities worth $85 for each of the remaining shares.

Even though the $8.8 billion offer is far in excess of Du Pont's. Wall Street experts said that there is enough antitrust risk in the Mobil offer that, barring an unexpected development -- such as a court victory for Mobil in its try to block Du Pont from ending its withdrawal period at midnight -- Du Pont is likely to hold on to most or all of the 49 million shares it now has and that shareholders that have not sent out their stock will ship it either to Du Pont or to Seagram.

Seagram is offering $92 a share and last Saturday began issuing checks for the 15.5 million shares it had in its control. Seagram said it will buy up to 44.35 million Conoco shares. The total value of its offer is $4.08 billion. The Seagram offer is scheduled to expire at midnight Wednesday.

The Justice Department dealt Mobil what appears to be a fatal blow when last Friday it asked Mobil for "more information" on which to judge whether the proposed Mobil-Conoco merger would pass antitrust muster. That request had the effect of delaying until Aug. 10 Mobil's ability to pay for Conoco shares. It also increased fears among Wall Street traders and Conoco investors that even the Reagan administration, whose antitrust attitudes are looser than its immediate predecessors, would try to block a marriage of two major oil companies.

Mobil is the nation's second-largest oil company, and Conoco is the ninth.

Du Pont, on the other hand, received antitrust clearance from the Justice Department today, as expected. Seagram, the big Canadian distiller, received clearance long ago.

Seagram touched off the battle for Conoco in late June, shortly after it lost an attempt to buy controlling interest in St. Joe Minerals Corp. Cash-flush since last year, when it sold off its U.S. oil and gas holdings to Sun Oil Co. for $2.3 billion. Seagram first made a friendly offer to buy 25 percent of Conoco's stock that was rebuffed by Conoco's board. Seagram then made a "hostile" offer directly to Conoco shareholders at $73 a share.

Conoco found a White Knight in Du Pont, who made its first offer on July 6. Shortly after that Mobil, which has arranged a $6 billion line of credit with its banks, jumped into the fray. The offers have escalated since then.

Du Pont, unlike the other two bidders, has said it will keep Conoco management intact.

Late today Conoco directors said they continue to think the Du Pont offer is the best for Conoco shareholders. The directors said it is "questionable" whether anyone offering stock to Mobil would ever be paid because of antitrust problems.

Mobil maintains that under 1968 Justice Department guidelines, it would not violate antitrust laws if it merged with Conoco.

Seagram, which is unlikely to gain the 51 percent of Conoco stock it needs to gain control of the company, will become the largest minority shareholder in a merged Conoco-Du Pont, with the ability to force some concessions. Wall Street sources speculate Seagram will swap the Du Pont stock it will receive for the Conoco stock it now owns for some of Du Pont-Conoco's assets, such as Consolidation Coal Co., the nation's second-biggest coal producer.

Although the price the companies are willing to pay for Conoco is high -- Conoco sold for about $49 a share last spring -- the worth of the company's oil reserves alone is about $140 a share.