When Edward Zorinsky switched from the Republican to the Democratic Party to run for the Senate in Nebraska four years ago, Republicans called him a "turncoat" and Democrats called him "an opportunist."

"I told them they were both right," says the Democratic senator.

Now, Zorinsky, who prides himself on being a maverick, is seriously considering doing the political unthinkable once again -- switching back and becoming a born-again Republican.

GOP leaders in the Senate couldn't be more pleased.

In the days before last month's election when the makeup of the Senate looked uncertain, they made overtures to Zorinsky and a number of other Democrats who tend to vote with the GOP much of the time. Among them were Sens. Harry F.Byrd Jr., the Virginia independent who caucuses with Democrats, David Boren of Oklahoma and Zorinsky's Nebraska colleague, J. James Exon, all of whom said no dice.

"I thought it was a joke. I never took it seriously," Exon says. "I've been a Democrat all my life, and I wasn't about to leave the Democratic Party under any circumstances."

But Zorinsky took it seriously, especially when the overtures continued after the election put Republicans firming in control of the Senate. The morning he returned from the election recess, Senate Minority Leader Howard H. Baker Jr. was on the phone at daybreak.

"He woke me up at home, and said he wanted me to drop by his office on the way to work," Zorinsky recalls.

Zorinsky did. "Baker just said that because of my voting record and past membership in the Republican Party, and because members of his side of the aisle respected me, he would like to have me back with them," he says.

He also heard from Sens. Paul Laxalt, Ronald Reagan's campaign chairman, Orrin G. Hatch (R-Utah), Bob Dole (R-Kan.), Mark Hatfield (R-Ore.) and Strom Thurmond (R-S.C.).

Zorinsky told them he has never been much of a party man. "I let them know I'd think it over and make a decision after the first of the year," he recalled the other day in a cloakroom off the Senate floor. "I never close the door on anything."

Then he added a caveat: "I told them I would do it only if it were in the best interest of Nebraska."

That translates in political terms this way: Zorinsky would like to cut a deal where he would get some small chunk of Senate power -- say a subcommittee chairmanship -- for switching parties.

As a Democrat, he holds two such slots -- chairmanships of an Agriculture subcommittee on farm credit and rural electrification and of a Foreign Relations panel on the western hemisphere. He is scheduled to lose these in the new Republican-controlled Senate.

A switch would cause Zorinsky no great emotional ordeal. He has few ideological or personal committments to the Democratic Party. In 1979, in fact, he voted against the party's positions 63 percent of the time, according to a Congressional Quarterly tally. Only Virginia's Byrd, who opposed the Democrats' position 74 percent of the time, opposed it more. Oklahoma's Boren opposed it 59 percent of the time.

The former Omaha mayor joined the party out of political convenience. He wanted to run for the Senate in 1976, but former senator Roman Hruska, and the party's leadership threw their weight behind another candidate in the primary.

Zorinsky chose to switch parties rather than fight.

Strangely enough, the Republican leadership in Nebraska does not much want him back. "Frankly, we're looking forward to running against him," says Dave Heineman, the party's state executive director.

This is one of the two biggest obstacles against Zorinsky's making a switch."I'm not accustomed to going to parties I'm not invited to," he says.

One reason for the opposition is that Zorinsky is toying with the idea of running against Republican Gov. Charles Thone in 1982.

The other roadblock for Zorinsky is how voters would interpret a change. "My concern is once you switch, people call you an opportunist and opportunist becomes associated with your name more than anything else," he says.