Ronald Reagan started his slightly dented Republican presidential bandwagon rolling again today by picking up the endorsement of a key moderate, Illinois Gov. James R. Thompson, and an estimated 33 additional Illinois delegates to go with it.

Reagan broke off campaigning in Indiana, where 54 GOP national convention delegates will be elected Tuesday, to fly to Midway Airport and hear Thompson extol him as a candidate who could draw Democratic and independent votes in November.

It was a welcome and timely endorsement for Reagan, who survived a scare Saturday night when early returns put him behind George Bush in the Texas primary. Reagan seemed somewhat shaken by the early Texas results at an Indianapolis press conference Saturday night, but he was happy today when he learned he had won the Texas popularity contest and 65 of the 80 delegates.

On the campaign plane to Illinois, Reagan told stories from his Hollywood days to reporters while aides looked forward to virtually clinching the nomination in Tuesday's primaries in Indiana, Tennessee and North Carolina.

Reagan picked up 135 delegates Saturday, including the Texas delegates and others chosen in caucuses in Arizona, Oklahoma, Missouri and Guam. The Reagan count now gives him 736 of the 998 delegates needed for nomination, and his strategists estimate he will acquire at least 100 more delegates in the Tuesday primaries and caucuses Monday in Colorado.

This would enable Reagan to go over the 1,000-delegates in California's winner-take-all primary June 3.

But the confidence within the Reagan camp does not run as high as the candidate's delegate count. Because of heavy earlier expenditures, the Reagan campaign has almost no money left for advertising in the remaining primaries and must try to match Bush's media spending with personal appearances by Reagan.

"We're like a ship dead in the water with submarines all around hoping that no torpedoes will hit us," said one aide.

Reagan aides also are worried about overconfidence, which some say afflicted them in Texas.

"There's no point in claiming victory until you really have it in the bag because then the people in California go to sleep, and you don't want that," Reagan chief of staff Edwin Meese said today.

The continued presence of Bush in the race already is forcing changes in the Reagan schedule. Even before the Texas primary, the campaign scrubbed scheduled appearances this week in Wyoming and Nevada, which are considered safe Reagan states, in favor of rallies in Maryland and Nebraska, where Bush is contesting upcoming primaries.

In this context, Thompson's endorsement was particularly welcome. He delivered it at a joint press conference where he described Rreagan as "the overwhelming choice of my party" and attacked President Carter, who he said had "turned the American dream of getting ahead . . . into a nightmare."

Reagan won the pivotal Illinois primary March 18 in a threeway race with Bush and John B. Anderson. Thompson's endorsement means that uncommitted delegates and others who will be chosen this week by the state party will go to Reagan.

Thompson said he wanted nothing in return for his endorsement. Another evaluation, however, came from Reagan state organizer Donald Totten, an old Thompson foe.

"Thompson wants to be chairman of the delegation worse than Reagan wants to be president," Totten told reporters today.

Based on his endorsement, Thompson is considered certain to get his wish.