TEXAS, THE NATION'S third most populous state, will send 80 Repulicans and 162 Democrats, as delegates, to the national conventions this summer. The Republicans will hold a variation of the old-fashioned "winner-take-all" primary today, while Democrats will hold both a primary, of sorts, as well as more than 5,000 precinct caucuses.

Because Texas allows no registration by party, Texas Republicans (as well as Democrats) are really a legal fiction. That has not deterred the GOP from conducting a presidential primary in which the candidate who receives a majority of the popular vote in each of the state's 24 congressional districts wins all three district delegates. The remaining eight delegates will go to the candidate who wins a majority of the statewide vote. After all the proportional representation of the Democratic season, where winning a major state may mean one more delegate for the victor than for the loser, Texas Republicans have put a welcome premium on coming in first.

What the Republicans have simplified, the Democrats have complicated. The first event on their schedule today is a "beauty contest" primary in which no delegates will be chosen. But in order to participate in the real delegate-selection process, which begins in the afternoon, the Democratic voter must be able to prove that he or she has first voted in the non-binding beauty contest primary. District conventions will follow the precinct caucuses and on June 20, Texas Democrats will meet in a state convention.

The favorite on the Republican side is Ronald Reagan. Four years ago, Mr. Reagan won every one of the Texas delegates over Gerald Ford. This time, against George Bush, Mr. Reagan has the all-out support of his vanquished presidential rival, former Texas governor John Connally. Mr. Bush, who has less momentum and more money than he had earlier, is campaigning vigorously and spending generously to overcome the Reagan lead and to avoid embarrassment in the state where he actually lives and votes.

But to win in Texas requires both time and money in abundance. It is further from Brownsville, Tex., to Amarillo, Tex., than it is from New York City to Chicago. To reach the state's 5-million-plus registered voters in 254 counties, a candidate is almost forced to buy time on one of the state's 55 television stations or 420 commercial radio stations.

While the rest of the country seems afflicted with collective self-doubt and economic recession, Texas remains both aggressively optimistic and increasingly affluent. Texas reminds a visitor of the United States in the decade of the '50s, and the vote today should provide some clue as to where Texas' 26 electoral votes will end up in November 1980.