The Democratic Texas Legislature is about to take one of the single most important actions of the 1980 presidential race -- one that is certain to have major influence on who the Republican nominee will be.

At the same time, the legislature could alter the campaign plans of candidates of both parties, diminish the importance of some other state's primary and give to Texans significant new clout in presidential politics for years to come.

What the legislature specifically faces is whether to hold presidential primaries in the state -- and when.

How it answers those questions is expected to go a long way in determining the fate of two Texas Republican presidential hopefuls -- former governor John B. Connally and George Bush -- and the fate of an adopted favorite son, Ronald Reagan, who swept a 1976 Republican primary here 2 to 1 over then-President Gerald Ford.

It could also "impact the dickens" out of President Carter's re-election bid, as one Democrat here puts it.

Moreover, current proposals would place the Texas primary on March 11 -- a bare two weeks into the 1980 primary season. Thus Texas could well overshadow the historically important Florida primary scheduled for the same day, because Texas would offer candidates in both parties more delegate votes toward nomination.

Not that the legislature, which has never been above tinkering with state election laws to help a good Democratic cause, is trying to do all of that. Things just might work out that way as conservative Democrats indulge in the politics of survival -- from the courthouse to the statehouse.

It is because of such tinkering that the legislature finds itself in today's bind.

Traditionally, the state chose delegates to the national political conventions at state conventions. But for 1976, to accommodate the dreamy presidential ambitions of a fellow Texas Democrat, U.S. Sen. Lloyd Bentsen, the legislature voted to have, in 1976 only, presidential primaries.

Bentsen lost miserably. More importantly, an estimated 200,000 conservative Democrats were drawn to the May 1 Republican presidential primary to vote for Reagan. But they were then ineligible to vote in the state Democratic primary held the same day As a result, some conservative Democratic legislators got bumped off by more liberal challengers.

Now the legislature is faced with strong pressures and court threats from Republicans who want another primary in 1980, but the Democratic leadership is trying to find a way to prevent a repeat of 1976.

Their answer to be presented next week: a permanent law setting a presidential primary in March and a state primary much later. And make each primary open to all voters, regardless of their party affiliation.

Lt. Gov. Bill Hobby, who presides over the state senate and who wants an early primary to give Texas more impact, and House Speaker Bill Clayton, who gets even rather than mad when people challenge him, expressed confidence the measure will pass.

They expect opposition, however, over allowing voters to cross back and forth between parties, voting Republican, for instance in the presidential primary and Democratic in the state primary.

Gov. Bill Clements has threatened to veto moving the state primary later in the year, as now proposed. It is now held in May.

Observers here think that an early presidential primary would mean that Texas alone could eliminate or irreparably damage one or two candidates among Bush, Connally and Reagan.

That is because both Bush and Connally, as Texans, should be expected to do well in their home state. And Reagan, whose organization here is second only to the one in California, might as well be Texan.

Bush -- lesser known, more in need of time to build and having lost two previous statewide elections in Texas -- is viewed as the most vulnerable to an early primary, athough it is known that Connally has fears of taking on Reagan in Texas and Florida at the same time.

Asked if a Texas loss to Reagan would virtually end his chances, Connally said: "Might." But on the other side, a Connally win, he said: "Might end his."

Reagan, generally viewed as the frontrunner in Texas and nationwide, could be vulnerable if he lost or barely won here, after such a stunning victory over an incumbent president in 1976. On the other hand, Reagan victories on the same day in Texas and Florida, where he did well but lost in 1976, could prove insurmountable to others.

Indeed, the Reagan stretegy depends on winning big and winning early.

What no can foresee, it seems, is an early Texas primary that wouldn't have all of the candidacies of Bush, Connally and Reagan. And whatever primary package emerges from the state house here, it seems certain an early presidential primary will be part of it.

For President Carter, who carried Texas in 1976, the impact is less clear, depending on who any challenger would be. But he has angered both liberal and conservative Democrats in Texas, and the president would probably be safest with a state convention -- as preferred by Democratic National Chairman John C. White, himself a Texan.

Whatever the impact on given candidacies, an early Texas primary, next year and in presidential election years to come, alters the campaign landscape for everyone.

Campaigning in the state is enormously expensive and time-consuming.Texas is the third most populous in the nation, with a score of major cities and media markets as far apart as New York is from Chicago, and state-wide races are more like national campaigns.

In between these major population centers are the widely scattered rural pockets where voters are less numerous, but where they turn out in greatest proportion and cannot be written off.

Not surprisingly, then, Texas' statewide races in 1978 were among the costliest in the United States, and campaigns lasted more than a year.

Thus candidates, pressed for time and money -- the Florida, Texas and Illinois primaries would fall in a one-week period -- could be forced to husband their resources by opting out of other state primaries. Or they may choose to pass up, at their peril, one of those delegate-rich states.