On the night of Nov. 6, as President Reagan was rolling toward one of the biggest landslides in American history, no one anticipated that a seesaw election in southwest Indiana might threaten the president's second-term agenda.

But as the House convenes today to begin voting to seat Democrat Frank McCloskey as the four-vote winner, Republicans are on the brink of open warfare. They say the Democrats stole the seat, and they want a new election.

What happened? The simple answer is there is no simple answer. One election, one state-sponsored recount and one federally supervised recount have left the voters of the 8th District with four conflicting vote counts and two different winners.

The argument that threatens to bring the House to a standstill centers around 32 absentee ballots. They appear invalid under Indiana law, were never counted for president or any other office on election night or since and have not been fully examined by federal auditors. Republicans say that they should now be counted in the name of fairness and consistency.

Both sides agree on some facts:

* McCloskey led by 72 votes on election night.

* A counting error in one county on election night eventually gave Republican Richard D. McIntyre a 34-vote victory. He was certified as the winner by Indiana's secretary of state, a Republican.

* House Democrats refused on Jan. 3 to seat McIntyre.

* A state recount enlarged McIntyre's margin to 418 votes, bringing him a second certification. But roughly 4,800 ballots counted on election night, many in predominantly black precincts, were declared invalid.

Enter a House task force.

The task force was the creation of the Democrats and the House Administration Committee. Its three members included two Democrats -- Rep. Leon E. Panetta (Calif.), the chairman, and William L. Clay (Mo.) -- and one Republican, William M. Thomas (Calif.).

The task force had the responsibility for setting the rules of counting and hiring a team of auditors to carry out the recount. Much of its work, especially in the early stages, was bipartisan. For example, 20 of the 22 rules used for counting were adopted unanimously.

The task force hired James H. Shumway, state elections officer in Arizona, to head the recount team and used auditors from the General Accounting Office (GAO) to conduct the count. "It was all handled in fairness and objectivity by all our people," Shumway said in a telephone interview yesterday.

Right up until the end of the counting, everyone thought McIntyre would win. But a couple of things changed all that.

One was tally errors -- mistakes that election clerks had made in recording the vote. The big one came in Daviess County, where 10 McCloskey ballots were mistakenly put in McIntyre's pile on election night and never discovered. The result was a 20-vote shift to the Democrat.

Another was unknown ballots. By the time the GAO auditors were making their final counts, the ballots had become familiar to partisans from the Republican and Democratic campaign committees who were monitoring the counting.

But in the final week, seven previously unknown and uncounted ballots were discovered in Greene County -- and all were for McCloskey.

Another problem confronting the task force was "ghost voters." This occurred in a number of precincts where the number of votes exceeded the number of voters who signed the register.

Because there were no allegations of fraud, the task force decided not to reconcile the differences. One way they could have was to reduce proportionately the vote totals for both McCloskey and McIntyre. Had they done this, McCloskey would have benefited.

But the real controversy came over how to handle an unexpected problem involving some absentee ballots. The GAO auditors had discovered 62 completed absentee ballots that had not been notarized or witnessed as required by Indiana law.

Ordinarily, such ballots are invalid and are to be kept by the county clerk. In this case, clerks in a number of counties erroneously sent them to the precincts to be counted.

A total of 52 of the 62 ballots were opened on Election Day and tossed into the pile of regular ballots. There was no way to retrieve them.

That presented a new question: What should the task force do with the 10 unnotarized or unwitnessed absentee ballots that were sent to the precincts and not counted?

On a 2-to-1 vote, with Democrat Panetta siding with Republican Thomas, the task force voted to count them. They went 6 to 3 for Republican McIntyre (with the 10th not voting in the House race).

At the same April 10 meeting, the task force discussed the existence of some unnotarized or unwitnessed absentee ballots still with county clerks, who had properly not sent them to precincts.

The task force decided to send affidavits to the clerks to ascertain whether those absentee ballots had been kept under the same kind of security as other ballots.

Thomas says this was a preliminary step before deciding whether to count them. Panetta says the task force never intended to count these "invalid" ballots and believes that Thomas wanted the affidavits for protection against Republican charges that valid ballots had gone uncounted.

On the night of April 17, Panetta and Thomas met privately in Panetta's office to go over the agenda for the final meeting of the task force, scheduled for the next night.

By this time, McCloskey had slipped into a three-vote lead with all counties recounted by the GAO. Democrats were stunned and privately a little embarrassed. Republicans were livid.

Thomas says his discussions with Panetta that night led him to believe that Panetta was prepared to count those ballots if security proved adequate.

Panetta's memory is different. "I told Bill I didn't want to bring those in," Panetta said yesterday. "I said, 'I don't see a basis on which to do that.' "

The next night the task force reconvened in Indiana. Results from the clerks' affidavits revealed: In nine of the 15 counties in the district, clerks had sent all such absentee ballots to the precincts. In two others, clerks could not state with certainty that the absentee ballots had been kept under similar security as other ballots.

That left four counties and, according to Republican counters, 32 absentee ballots that the clerks said were adequately secured. In general, the ballots were in Republican counties.

In one of those four, however, recount director Shumway had doubts about the security.

Still, Thomas argued that, having counted the "invalid" absentee ballots improperly sent to the precincts, the "invalid" absentee ballots properly kept by the clerks should also be counted. But on a 2-to-1 vote, he lost. Thomas, crying "rape," said Panetta had made a "distinction without a difference."

Thomas says Shumway has since said in public testimony that, given similar security and handling, those votes should have been counted.

Shumway, in a telephone interview, says he was answering a hypothetical question. On the specific question of the 32 votes, he said he would give no opinion.

"I neither recommended that they do or that they don't," he said. "They're the rule-making body. They were from the outset."