Mobil Corp., engaged in a hot bidding war for Conoco Inc., said today it is "considering" increasing its cash offer for about half of Conoco's shares from $90 to $95, to equal the amount E.I. duPont de Nemours & Co. has said it will pay for 40 percent of Conoco's stock.

If Mobil should boost its offer, the total value of the cash and securities offer for Conoco would be $7.96 billion. The value of the Du Pont offer is about $7.2 billion today.

Meanwhile, Conoco -- sought not only by Mobil and Du Pont but by Seagram Co. as well -- filed suit in Washington to block the Mobil offer, alleging that a merger between Mobil and Conoco would violate the nation's antitrust laws. Mobil is the nation's second-biggest oil company, with annual sales of $59.5 billion. Conoco is ranked ninth, with sale of $18.3 billion.

Mobil has argued that the two oil companies do not violate antitrust guidelines set down in the Johnson administration. The two companies are not big enough factors in any market to make a merger anticompetitive, Mobil has said.

In Washington, meanwhile, Rep. Peter Rodino (D-N.J.), chairman of the House Judiciary Committee, charged that the merger wave "could slow down our nation's economic growth" and announced plans to hold hearings this fall on antitrust policy with respect to corporate mergers.

In an opening statement before a hearing on corporate initiative, Rodino, whose committee overseas antitrust policy, said economists "agree" that "increased efficiency is unlikely once a corporation exceeds a certain size" and criticized the Reagan administration's general plans to rewrite merger guidelines.

"By failing to announce specifically what level of concentration it finds unobjectionable, the administration may have created a void into which unguided and aggressive corporate powers will quickly and eagerly plunge," Rodino charged.

Large mergers could limit the nation's economic growth, Rodino said. "If cash in corporate treasuries is not invested to promote long-term growth, but is spent instead to acquire the assets of other firms, a major potential source of investment will be dissipated," he said.

Conoco, in its suit today, said the companies overlap in a host of oil and oil-related markets, that the competitive "impact" of Conoco's operations is "substantially greater" than percentage sales of gasoline suggests and that a Mobil-Conoco merger would trigger other takeovers of so-called second-tier oil companies by the giants.

Texaco, the third-biggest, has arranged a $5.5 billion line of credit, while Gulf, the fifth-largest, has a $5 billion credit line. Both companies are rumored to be searching for acquisitions.

Conoco has embraced the Du Pont offer and has fought both the $3.77 billion Seagram bid for 44.35 million Conoco shares and the Mobil offer. The fight to take over Concoco is the costliest and most fiercely contested in corporate history.

Mobil late today went to court to try to block a portion of the agreement between Conoco and its favored suitor, Du Pont. Concoco has given Du Pont an option to buy up to 15.9 million shares of stock in the Conoco treasury at $87.50 a share.

Mobil said that if Du Pont were permitted to exercise that option, the chemical giant could be able to achieve "a bare majority of Conoco shares" -- even if only 40 percent of the 86 million shares in public hands were tendered to Du Pont. If Du Pont gained control of the company that way, Mobil alleged, Du Pont could then "proceed to acquire 100 percent of Concoco at a price lower than that being offered by Mobil."

Mobil's current offer, which the company might revise, is $90 a share for at least $43.5 million shares of Conoco, to be followed by an exchange of Mobil securities (preferred stock and debentures, which are a type of bond) worth about $90 a share, for the rest of Cooco's stock. There are about 86 million shares of Conoco outstanding.

Du Pont will pay $95 a share for 34.44 million shares of Du Pont common stock for each of the remaining shares. Du Pont's offer (which is a revision of its original offer) had a total value of about $7.4 billion when it was made last week.