Amid intense political maneuvering and accusations of bad faith, the Democratic-controlled House yesterday headed for a showdown today on President Reagan's plan to provide $100 million in U.S. aid to the Nicaraguan contras.

While Reagan made a last-minute televised appeal for his proposal, House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) accused the White House of engaging in "a small, cheap and mean" political tactic in asking that Reagan be allowed to address the House on his plan to aid the counterrevolutionaries.

O'Neill, charging that this was a ploy meant only to embarrass him, said he had offered to postpone the vote until Thursday so the president could address a joint session of Congress today.

"The president is an excellent communicator," O'Neill said. "If his policies in Nicaragua matched his eloquence, they would not be so overwhelmingly unpopular with the American people."

White House spokesman Larry Speakes, accusing O'Neill of delaying tactics, rejected the joint-session offer.

In his speech from the Oval Office, which only the Cable News Network among the major television networks agreed to broadcast live, Reagan took a conciliatory tone that was in sharp contrast to the strongly partisan administration rhetoric that preceded the first House vote on the contra aid issue in March. The president acknowledged the "honest fears" of opponents of his Central America policy, but appealed for "bipartisan support" to prevent further delays in aiding the contras. Details on Page A10.

Republican and Democratic leaders alike predicted a closer vote than the 222-to-210 tally by which the House in March rejected Reagan's original proposal for $70 million in military aid and $30 million in nonlethal "humanitarian" assistance for the contras.

"I think it will go right down to the wire," O'Neill said.

The key vote today is expected to be on a Republican-backed proposal to provide the contras with an immediate $40 million in aid, including an unspecified amount of military assistance. Under this proposal, the contras would receive another $20 million on Oct. 15 and the final $40 million next Feb. 15.

Congress could block the later payments by passage of a disapproval resolution, but this would be subject to a veto by Reagan.

The GOP plan will be offered as an alternative to a proposal crafted by a group of moderate Democrats headed by Rep. Dave McCurdy (Okla.), who prevailed yesterday over a group of liberal Democrats trying to eliminate all contra aid. The McCurdy plan, which has the support of Democratic leaders, would provide $30 million in humanitarian aid immediately but make payment of the $70 million in military assistance subject to a second vote by Congress on or after Oct. 1.

The administration strongly opposes a second congressional vote to free the military aid, which is the key difference between the competing proposals.

As part of the intensifying feelings on the issue, four former contras charged in an interview yesterday that any aid given to the current rebel leadership will only be siphoned off and wasted through corruption.

"There is so much anger among the contra troops now over being robbed that there is a danger of an internal coup," said Alberto Suhr, a former Sandinista school construction director who claims to have been a counterintelligence agent for the largest rebel group, the Nicaraguan Democratic Force (FDN), from 1983 to 1985.

Jose Francisco Cardenal, a member of the first FDN directorate who said he was ousted in 1982 after complaining about its corruption, said "the aid should pass, but if it goes to this leadership it will be lost."

Also yesterday, the administration's chief Central America policy spokesman said that charges from the former contras "are coming from people who were kicked the hell out of the FDN" for supporting former dictator Anastasio Somoza.

Elliott Abrams, assistant secretary of state for inter-American affairs, summoned reporters to his office to condemn what he called "hysterical charges" from congressional investigators that previous contra aid funds might have been illegally diverted to offshore bank accounts, elusive corporations and the Honduran armed forces.

He denounced the House Foreign Affairs subcommittee on Western Hemisphere affairs and its chairman, Rep. Michael D. Barnes (D-Md.), for "sheer out-and-out McCarthyism" in demanding a State Department response to the allegations on only 24 hours' notice and without providing all the necessary data. "That's not an inquiry; that's a smear," Abrams said.

Barnes responded that the administration is "stonewalling, and I believe that they are engaged in a cover-up . . . . We have given them access to everything we have." He said it is "clear they don't want to be subjected to questions on this program before the vote."

As all sides fully aired their views, accusations continued over the White House request for Reagan to address the House on the issue, which O'Neill on Monday rejected as an "unprecedented" departure from the tradition of presidents addressing joint sessions of House and Senate. At the center of the controversy were two men who grew up in the same working-class neighborhood of Cambridge, Mass.: O'Neill and White House chief of staff Donald T. Regan.

Interviewed by Cable News three hours after Reagan's speech, O'Neill was asked why there appeared to be "bad blood" between him and Regan, who made the request for the president to speak to the House.

"I stayed with the people," O'Neill replied. "He's moved an awfully long way from the other side of the tracks."

According to O'Neill, the White House was warned by sympathetic Democratic lawmakers that the request would "embarrass" the House Democratic leadership and almost certainly be rejected.

"They took the position, fine, now we can embarrass O'Neill," the speaker said. Blaming the episode on Regan, he added, "I don't think the president of the United States would have participated in a thing as small, cheap and mean as this."

O'Neill's version of the dispute was supported by Rep. Buddy Roemer (D-La.), a supporter of the administration's contra aid package. Roemer was among a group of lawmakers who met last Friday with national security affairs adviser John M. Poindexter to discuss strategy for the upcoming vote.

Roemer said O'Neill should have granted the White House request, but added:

"If there is any politics here, it is the White House, not the speaker, who is at fault . . . . The bipartisan aid package is a good one, but it is not helped by political demagoguery on the part of an often incompetent White House."