It was just a word change, so casual and offhand that it was probably noted by few of the millions watching President Reagan's press conference Wednesday night. When he was winding up an answer about the recession, he said the signs indicated that "we are bottoming out, and I believe we're safe in saying that we think there's going to be an upturn in the second half of the year."

The second half of the year? All through the winter, the administration view has been that recovery would begin in the second quarter of the year. That change of words is freighted with political gloom for Republican candidates.

It is a concession on the part of the always optimistic president that the recession that began last July will last at least a year before it begins to relax its grip. It means that the "tragedy" of which Reagan spoke for the unemployed and for farmers, builders, merchants and small businessmen being pushed to the wall will be prolonged. When the country goes to the polls in November, many will still be hurting, and for many more the pain will be a recent, searing experience.

Particularly is that likely to be true of the jobless. The history of past recessions clearly suggests that rehiring will lag behind the turnaround in sales and production. If the president is right in his forecast, the odds are good that unemployment will be at its peak level during the fall campaign.

What makes it worse for the Republican candidates is that there is now almost nothing that can be done to alter the circumstances they will face in November. At his press conference, Reagan ruled out any consideration of emergency measures to stimulate the economy, saying that similar efforts in the past simply bought short- term relief at the cost of escalating inflation. But even if he were inclined to try, the odds would be against any economic medicine's being felt in the system in the time that remains before Election Day.

The only stimulus in sight is the 10-percent tax cut scheduled for July 1. Administration economists hope that this boost to real income will trigger an upturn in spending that will signal and sustain a turnaround in the economy.

Whether the tax cut will be sufficient to overcome the drag of high interest rates is questionable. Families that have held off on the purchase of homes, cars or major appliances may still balk at the carrying costs of the loans they would need. But even if the tax cut works, the recovery is probably coming too late to reverse the gloomy prospects Republicans face.

The trend shows clearly in a set of polling numbers on voters' preferences for the November congressional elections. Last November, when the hope was that the recession would end with the departure of chilly weather, a Washington Post- ABC News poll gave the Democrats a 53-40 percent lead nationally. By early March that lead had widened to 19 points, 55-36.

That could easily translate to the 30-seat House loss that Sen. Paul Laxalt (R-Nev.), the president's closest political pal, said last week the GOP faces unless a quick budget compromise paves the way for a rapid fall in interest rates. No such compromise was even vaguely suggested by Reagan's press conference words.

The numbers have been deteriorating rapidly for the GOP as the recession has taken a heavy toll on public confidence in the Republican administration and what is widely believed to be a Republican-controlled Congress as well.

A Gallup Poll taken in February and published last week showed Republicans trailing the opposition in their handling of seven of eight major issues, the lone exception being national defense. Even the success on the inflation front, on which Reagan did some justifiable bragging, does not seem to be working to the benefit of his party.

No longer is there serious talk about Republican gains in November. The operative question is whether the losses can be held below the point that would make Reagan a lame-duck president two years early.