A coalition of conservative leaders yesterday warned President Reagan to expect an "extremely acrimonious" political backlash from "millions of conservative supporters" if he agrees to any cutoff of arms sales to Taiwan.

In a strongly worded statement, 28 conservative leaders urged Reagan to reject recommendations from former secretary of state Alexander M. Haig Jr. that the president sign a joint communique with the People's Republic of China limiting arms sales to Taiwan.

The group said many conservatives already believe the Reagan administration has abandoned Taiwan, and any hint of cutting off arms sales "could easily break apart the 1980 coalition that elected Reagan, which is already beginning to crack."

The statement was endorsed by the leaders of 28 groups, ranging from such New Right organizations as the National Conservative Political Action Committee and the Moral Majority to the Young Republicans and College Republicans.

Haig, in what may have been his final act as secretary of state, last week sent the White House language for a joint communique designed to settle a long-simmering dispute that has brought Sino-American political and strategic relations to a standstill for about six months.

According to Gary Jarmin, executive director of the American Council for a Free Asia, a lobby group supporting strong U.S. ties with anti-communist Asian states, Haig submitted two alternative proposals to Reagan.

One, Jarmin said, states that the U.S. renounces a policy of "long-term arms sales" to Taiwan. The other contains more emphatic statements that such sales "will eventually be terminated," he said.

Conservatives yesterday called these recommendations an "ultimate and humiliating blow to Taiwan."

"We cannot emphasize enough that such language is an inexcusable and total sell-out of Taiwan," the group added.

Several spokesmen said they think Reagan and his advisers underestimate the strong feelings among conservatives over the arms sales issue.

"There is a sense of anger over this that I haven't seen on any other issue," said Paul Weyrich, a leading New Right strategist and president of Coalition for America.

Jarmin told a news conference that if the president approves anything that even hints at a cutoff of arms sales "there is going to be such a vicious backlash that it will leave his Reagan's head swimming."

Vice President George Bush would be a particular target of conservative anger if arms sales are phased out, Jarmin added. "His fingerprints are all over this."

Reagan reportedly considered Haig's recommendations last Thursday before leaving for California, and sent the proposals back to the National Security Council for further work.

Reagan has been a longtime supporter of Taiwan and he assured Sen. Barry Goldwater (R-Ariz.) last month that he would not agree to a cutoff date for arms sales to the island nation.

But conservatives said they fear Chinese threats to downgrade relations with the U.S. may convince Reagan to agree to a vaguely worded communique committing the U.S. to eventually end arms sales.

The conservative leaders said they see "no harm" in downgrading relations. "In fact, such a downgrading would help to restore a more realistic and balanced China policy," their statement said.