President Carter today announced his intention to nominate Texas state Rep. Matt Garcia to head the Immigration and Naturalization Service, but Garcia surprisingly said he would have to think about it.

The episode left the White House with egg on its face even though Garcia's nomination has wide support in the Chicano community here.

The long-delayed announcement was timed to coincide with the opening of the Texas Democratic convention, where more than 60 Mexican-American delegates and alternates are participating. About two-thirds support Sen. Edward M. Kennedy (D-Mass.), and Carter-Mondale forces are working hard to win them over for what is expected to be a tough fall campaign in Texas.

The first word of Garcia's nomination came this afternoon when White House assistant Rick Hernandez gave the news to the Mexican-American Caucus. But Garcia, in a somewhat emotional statement to the group, said he would have to "agonize" on whether it was more important for him to take the INS job or stay in Texas politics and work to assure the election of a Democrat this fall.

It is unusual for a presidential appointee to ponder so publicly over a job once the nomination has been announced.

Garcia later said he was disturbed over the long delay in his nomination. The commissioner's post has been vacant since Leonel Castillo resigned in October. Garcia has been on the White House see-saw for several months.

"I was ready to go in March," Garcia said. But this afternoon he clearly appeared to be on the fence, and said he had "expressed reservations about it coming so late" to fellow Texan and Democratic National Chairman John C. White.

Asked why the nomination had taken so long, Garcia said, "You'll have to ask Washington why it was delayed. I was minding my own business."

Hernandez said Garcia's nomination had been tied up because an FBI check reported that one of Garcia's legal clients had jumped bail and Garcia was liable for it. That has since been cleared up.

Hernandez insisted there was a "90 percent" chance Garcia would take the job and said he would spend part of the weekend talking about it with Garcia.

The INS post is especially important to the Mexican-American community because the commissioner sets policies affecting millions of illegal aliens in this country.

"The Mexican-American community won't let him put off a decision," said Ben Reyes, a member of the Houston City Council. "We're not going to let him wait six months. There are key appointments to be made and he's the one who ought to be making them."

Hernandez expressed no chagrin at the course of events here this afternoon. "I knew he was going to respond this way," Hernandez said, adding that the White House nonetheless felt it important to demonstrate to the Mexican-American community that Carter wanted Garcia for the job.

"That's an important statement to make to the Chicanos of this state and the Southwest," Hernandez said.

Garcia denied that his reluctance to accept the job immediately had anything to do with the prospect of a Carter defeat this fall. But Hernandez said Garcia "is the leader of our campaign in the Chicano community and he's worried about what will happen if he drops out now."

Said Garcia, "I'm a belt-and-suspenders guy. I want both. I want to make sure Carter carries Texas."

Garcia said he was most worried that his Senate confirmation hearings might get delayed because of election year politics and wondered whether it was worth it to take himself out of Texas politics for a possibly fruitless venture in Washington.

Later tonight at the opening of the convention, Garcia said, "It's not a decision whether to take the job or not. I want to clarify that. If there is the likelihood that my nomination can go before the Senate soon, I'll leave for Washington immediately."