Republican leaders bluntly warned Secretary of State George P. Shultz yesterday that the Reagan administration's proposed arms sale to Jordan is in trouble in the Senate and that the White House, at best, will have to accept conditions on the sale to head off an embarrassing defeat.

Senate Foreign Relations Committee Chairman Richard G. Lugar (R-Ind.) said yesterday, after meeting with Shultz, that despite the warning the administration will go ahead with plans to send the$1.5 billion arms package to Congress Monday.

"The secretary indicated the president is going to send the request up. It's something that's not negotiable," Lugar said. He said President Reagan thinks that the sale of jet fighters, antiaircraft missiles and other advanced weaponry is needed to show support for Jordan's King Hussein, who has said recently that he would enter peace negotiations with Israel under certain conditions.

The meeting with Shultz occurred as opponents of the arms package disclosed yesterday that 72 senators -- 44 Democrats and 28 Republicans -- have agreed to cosponsor a resolution to block the arms sale. That number is sufficient to override a presidential veto.

"We all hope that the courageous efforts made by King Hussein to further peace in the Middle East will bear fruit and that all parties will be able to sit down and negotiate for peace in the Middle East," Sen. Edward M. Kennedy (D-Mass.), one of the chief sponsors of the resolution, said in a statement yesterday. "But until those direct negotiations are going on, any arms sale to Jordan is a mistake."

Unless both houses of Congress pass disapproval legislation, the arms deal would go into effect 30 days after formal administration notification.

The arms package also faces strong opposition in the Democratic-controlled House. A resolution of disapproval is likely to pass there, congressional officials said.

Reagan is expected to veto any joint resolution of disapproval. A veto can be overriden only by a two-thirds vote of both chambers.

Lugar said yesterday that the Senate Republican leaders, including Senate Majority Leader Robert J. Dole (R-Kan.), told Shultz that the administration stands a good chance of losing. Lugar said, "It's apparent you're going to have to find another course."

He said the other course the GOP leadership would pursue is to draft a resolution of approval that would allow the arms sale to go ahead only if certain conditions were met.

"My hope is that a resolution of approval, with certain conditions, would be more attractive than just shooting the thing out of the water," Lugar said.

He did not specify conditions, saying only that the Republicans told Shultz, "there has to be movement and hope in the peace process. There has to be a sense of progress. Face-to-face negotiations is the sina qua non of that."