Chris Craft/United Television and the Federal Communications Commission are the wildcards in determining the future of UPN if the CBS-Viacom merger is completed.

Sumner Redstone and Mel Karmazin say they want to keep the nearly 5-year-old weblet alive, but their plans hinge on whether the FCC will bless that element of the deal and how Chris Craft, Viacom’s 50-50 partner in UPN, reacts to the CBS-Viacom union.

“UPN is doing a lot better than it was. There’s no reason we can’t program it successfully, whether we can keep it or not,” Redstone said during Tuesday’s news conference unveiling the merger.

Redstone and Karmazin are headed to Washington today to plead their case to the FCC. The deal, as currently proposed, would violate the FCC’s cap on ownership of stations covering more than 35% of the country, and it also violates the ban on one company owning more than one national broadcast network.

The various scenarios unfolding around UPN made for a tense atmosphere Tuesday at the weblet’s West L.A. headquarters, although insiders were heartened by the support voiced by both Redstone and Karmazin.

“UPN has received strong assurances from its parent companies that they are fully committed to the network’s long-term operation and prosperity,” UPN said in a statement.

There was instant speculation Tuesday that Viacom would quickly jettison the ratings-challenged UPN if it proved to be a sticking point in winning regulatory approval of the deal. But one UPN insider added that the combined Viacom-CBS would have a strong interest in keeping the younger-skewing weblet afloat.

“Mel has always believed in the value of multiple distribution systems,” the source said. “At the end of the day, it’s about money with Mel. And if Mel believes he’ll make money with UPN, he’ll keep it.”

Insiders say Viacom brass have already approached Chris Craft about restructuring their UPN partnership. Ideally, Viacom would like to buy out Chris Craft’s 10 stations and its half-interest in the weblet.

At the same time, reflecting the uncertainty surrounding aspects of the Viacom-CBS merger, there’s talk that Chris Craft might acquire the Viacom-owned UPN affiliate stations that Viacom-CBS may have to divest to keep in line with the FCC’s rule limiting station ownership to 35% of U.S. TV households.

The combined CBS-Viacom station holdings reach about 41% of the country. But if the FCC is inclined to lift that cap to 45% or higher, as some have speculated, then Viacom-CBS could keep the bulk of their stations and buy out some or all of Chris Craft.

Yet the FCC could hew to its rule limiting the ownership of networks, which could force Viacom to lower its stake in UPN to 32% or less.

Sources say the working relationship between Viacom and Chris Craft has often been tense, and it’s not at all clear if Chris Craft would be willing to be flexible as CBS and Viacom work out the regulatory kinks of their union.

Chris Craft already had been seen as a takeover target in the wake of the FCC’s decision last month to relax its duopoly rules regarding ownership of more than one station in a market — the move that turned out to be the catalyst for the Viacom-CBS pact.

Chris Craft/United TV execs did not return calls seeking comment Tuesday. The stock price of its parent company, BHC Communications, soared $6.25 Tuesday on heavy trading volume to close at $139.25, well past its previous 52-week high of $135.

(Christopher Stern in Washington and Josef Adalian in Los Angeles contributed to this report.)