A Republican Nebraska legislator has switched to the Democratic party, giving Democrats an even split with the GOP in the state's unicameral legislature and possible control for the first time since it was created in 1937.

State Sen. James Pappas of North Platte was the fourth Nebraska GOP legislator to switch to the Democrats in the last year and a half.

Pappas made the switch on April 5, but made no announcement until he was questioned by reporters more than a week later. He originally was a Democrat but changed to the GOP in 1976 because he said he thought Jimmy Carter and Sen. Edward M. Kennedy (D-Mass.) were taking the Democratic Party down too liberal a road.

He said he switched back to the Democrats because of dissatisfaction with the Reagan administration's agriculture policies, the president's apparent indifference to reducing federal budget deficits and what he described as the state Republican Party's "hard-line, pro-business, pro-wealthy" conservatism.

The Nebraska legislature now has 24 Democrats, 24 Republicans and one independent, state Sen. Ernie Chambers of Omaha, another maverick, who registered as a Democrat last year so he could vote for the Rev. Jesse L. Jackson in the presidential primary. Tie-breaking votes are cast by Lt. Gov. Donald McGinley, a Democrat.

This shift could set back the Republicans' hopes of winning control of enough state legislatures by 1990 to offset the almost 2-to-1 Democratic margin, and the resulting advantage in redistricting congressional seats after each Census.

Both parties speculate that Pappas made the switch because he wants to run for Congress against Republican Rep. Virginia Smith. Another recent GOP defector, Tom Vickers, unsuccessfully ran last year against Smith, who took 83 percent of the vote.

Neither Democrats nor Republicans professed surprise at Pappas' move because of his maverick reputation.

"The history of the Democratic Party in this state is that when the farmers get into trouble they look to the Democrats for leadership," said Democratic State Chairman Tom Moynihan.

Republicans tried to put the best face on it.

"I'm disappointed when anyone leaves the party," said GOP state Chairman Norman Brashear II. "Maybe he was never a real believer in the Republican philosophy . . . . Republicans know we've lost on this, so we have to buckle down, work harder and go onward and upward."

Pappas said he thought both major parties had changed substantially since his first party switch nine years ago, with the Democrats becoming "more centrist and moderate."

"A lot of people don't think there's room in the Republican Party for moderates and liberals," he said. "You have to be conservative, hard-line, pro-business, pro-wealthy and against social programs. The party doesn't cater to the mainstream and I don't like the slowness in reducing the big deficits. The only ones who benefit from them are the rich, people with money who draw interest.