Negotiators for Potomac Electric and Power Co. and Local 1900 of the International Brotherhood of Electrical Workers reportedly were making "some progress" in resolving economic differences in contract bargaining last night.

But sources close to the talks said some major obstacles remained to reaching an agreement by 6 p.m. today, when the union has the right to call a strike or continue talking.

Last night, there was little indication of the route the union might choose.

"They're meeting right now, and they probably will be meeting late into the night," said one bargaining source, who asked that name and affiliation be withheld. "We're not far enough along in this thing to say anything, one way or the other, about any strike."

Speculation about a possible strike by Pepco's 3,500 unionized workers began last week when Local 1900 members voted 2,886 to 6 to authorize a walkout.

Pepco notified the D.C. Public Service Commission late yesterday that it may have to seek delays in a hearing scheduled for Friday because the company's executives could be tied up with the union dispute.

Strike votes are routine collective bargaining actions, usually taken before negotiations begin. The IBEW vote came three months after the Pepco-IBEW talks started, and three days after the union's three-year contract expired May 31.

Strike speculation increased at 6 p.m. Tuesday, when Local 1900 officials exercised another contractual right by giving the company 48 hours' written notice of the union's intention to terminate formally its old agreement, which had been carried forward daily since May 31.

Union officials emphasized yesterday that service of the notice did not establish a firm strike deadline. They said they hoped the notice sent a message to company negotiators that the union wants to speed up action on a new contract.

Here are the outstanding issues, as they appeared late yesterday:

* Job security--The company says it wants to reclassify jobs "as part of a review that will affect a substantial number of employes, and for which the company is prepared to allocate a substantial amount of money." The union says the company wants the right to reclassify jobs unilaterally, thus undermining job security and reducing future union bargaining strength.

* Wages--The company has offered increases of 8 percent in the first year, 7 percent in the second, and 6 percent in the third year of a proposed three-year contract, which the company says would yield a "compounded wage increase of 22.5 percent" over the three years.

The union has not revealed publicly its wage counterproposal. But the IBEW says the company's figures do not add up. Instead, the company's proposal would yield a 15.3 percent to 15.9 percent wage gain over three years when other "takeaway" provisions in the contract are factored in, the union claims.

* Sick days--The company wants to crack down on what it says is a growing abuse of sick leave, but the union says the company's proposal would penalize all of the innocent in pursuit of the guilty.