Vice President Bush today returned to his adopted home state of Texas, to the congressional district that first sent him to Washington 18 years ago and ended his long campaign odyssey with the declaration: "We want to unite America as she has never been united before."

Bush arrived here after stumping in El Paso, Amarillo and Denton for Republican challengers to Democratic Reps. Tommy J. Vandergriff, Ronald Coleman and Jack Hightower. He did so as the multimillion-dollar GOP coattails crusade was flooding the men's districts with last-minute television advertisements and direct-mail literature.

Bush traveled to 21 congressional districts in the past week as part of a Republican effort to convert an anticipated landslide by President Reagan into substantial gains in the Democratic-dominated House, which has blocked many of Reagan's programs since 1982.

Ron Kaufman, liaison to Bush from the Reagan-Bush '84 committee, said the GOP campaign is spending $50,000 on ads and direct mail in each of the districts that Bush visited.

Today, a day after Walter F. Mondale denounced Reagan and Bush for neglecting the themes of decency and compassion, Bush told a welcome-home rally here that Reagan wants "an America that commands the respect of other nations not because of her strength alone but because of her decency and compassion."

Recalling his favorite encounters during the campaign with "unsung American heroes," Bush extolled the work of Detroit antipoverty activist Father William Cunningham and that of crimefighters and patriots. Cunningham has criticized Reagan's social-spending cuts.

Bush was greeted here with shouts of "Kick Some Ass," a reference to his off-color remark after the vice-presidential debate, and posters proclaiming, "Texas Loves George" and "Ride 'em Reagan." A high school band belted "The Eyes of Texas Are Upon You."

Earlier, at North Texas State University in Denton, several thousand students cheered and waved placards with such slogans as "Kick Ass George" and "Baseball, Hot Dogs, Apple Pie and Reagan-Bush."

Bush, who this year has encountered the most critical media coverage of his career, hit Texas in an upbeat but battered mood Sunday night and told an El Paso crowd that the campaign was "tough, it's grueling . . . and sometimes it can be just plain ugly."

Although the Democratic House incumbents are leading in the three Texas districts where Bush campaigned, Reagan is about 30 points ahead of Mondale in Texas, according to Kaufman. In 1980, he carried the districts with majorities ranging from 58 percent to 69 percent.

Bush, who has stumped for Reagan in 97 cities and 32 states since Labor Day, said he ends each "grueling day" feeling: "Thank God, I'm American, and thank God I live in the best, freest, most decent country in the world."