PHOENIX, Ariz. (AP) _ It's still the Wild West in Arizona, where a Republican-turned-Democrat is running an independent campaign for governor, challenging his adopted party's candidate and a Republican who beat President Reagan's personal choice.

In the midst of the disarray, experts say they aren't sure how businessman Bill Schulz's last-minute campaign will stack up against those of Democratic nominee Carolyn Warner and Republican Evan Mecham.

Schulz, who dropped his Democratic candidacy a year ago to care for an ill daughter, rejoined the race last week as a draft campaign gathered nearly 32,000 petition signatures to put him on the ballot.

That was far more than the required 10,259 valid signatures, but his former friends in the Democratic Party were considering a formal challenge of the signatures' validity.

Democratic leaders, facing a Friday deadline for the challenge, said Monday that their preliminary check of signatures indicated 75 percent or even more of them appeared invalid.

Some signers didn't appear to be registered voters, and at least one petition appeared to have every signature written in the same hand, they said.

However, Schulz spokesman Mark Regan scoffed at the criticism, estimating no more than ''maybe 5 percent they could have problems with.''

Mecham, an auto dealer, and Mrs. Warner, the state schools superintendent, are saying publicly they welcome Schulz into the race and expect him to take votes away from their opponents, and not them.

But state Democratic Party leaders have blasted Schulz for his behavior two weekends ago when he was considering whether to enter the race. Party leaders contend he promised to stay out, then changed his mind within a day.

A newspaper poll - taken before Schulz had even announced he was willing to be drafted - gave him 18 percent of the vote compared with 29 percent for Mrs. Warner and 28 percent for Mecham. Without Schulz in the race, Mecham would have a 4 percent lead over Mrs. Warner, according to the poll.

''It is so volatile at this moment that I think nobody could make predictions,'' Mrs. Warner said.

The confusion also could affect other state and local candidates who may have been counting on using somebody's coattails to win in November.

Things looked entirely different just a few weeks ago. The Republicans had hoped to take over the governor's office with a strong general election candidacy by longtime state House Majority Leader Burton Barr, who was personally asked by President Reagan to enter the race and who seemed headed for a sure primary victory.

But Barr lost a shocker to Mecham in the Sept. 9 primary. The postmortem consensus was that he took Mecham and the voters for granted by not responding to the auto dealer's allegations that Barr had a conflict of interest in his legislative and business dealings.

In the first days after the primary, the GOP seemed to be the party that was in trouble, while Mrs. Warner was confidently predicting a large crossover vote by unhappy Republicans would give her victory in November. Some Republicans said privately they weren't willing to line up behind Mecham, a strong conservative.

But the handful of suburban Phoenix business people who decided to draft Schulz into the race made it clear they also weren't satisfied with Democratic voters' choice of Mrs. Warner over businessman Tony Mason.

Mrs. Warner's campaign spokesman, Rick DeGraw, claimed sexism, saying the group was ''scared to death to have a woman as governor,'' although Mrs. Warner said she did not want to consider the possibility that her sex was a factor. If elected, Mrs. Warner would be Arizona's first female governor.

Schulz himself had said gathering enough petition signatures in a week could be an impossible task, but the signatures poured in for the former apartment magnate who made his name in 1980 by almost defeating Sen. Barry Goldwater, R-Ariz.

As the signature-gathering continued, Democratic leaders - including Gov. Bruce Babbitt, who had backed Mrs. Warner's primary opponent - staged two news conferences to drum up support for the woman officially running on the party line.

However, Schulz has disputed the notion that ''party loyalty is more important than anything else,'' adding that many of those Democrats were backing him for governor in 1985 when he was the odds-on favorite to win the party nod.

Schulz said Democratic party leaders had begged him not to re-enter the race, saying they feared his candidacy would hand the election to Mecham. State party Chairman Sam Goddard warned that a spoiler has no future in the party, and Schulz acknowledged that after the election he would be either ''governor or goat.''

Schulz quit the Democratic race in September 1985 because hi 26-year-old daughter suffered an illness which he would not specify. He told reporters last week that she had suffered a ''clinical acute depression'' and had attempted suicide, but that she had improved markedly this summer. He promised to fulfill a gubernatorial term even if she suffered a relapse.