Walter F. Mondale today climaxed an eleventh-hour appeal to blacks and Hispanics, the most loyal foot soldiers of the Democratic Party, by rallying more than 25,000 jubilant Texans with a prediction that "we're gonna win it."

"On Tuesday, the pollsters and the Republicans are in for the biggest surprise of their lives," Mondale told a largely Hispanic crowd that packed the bleachers and spilled off the infield of Buccaneer Stadium.

"For four years, they did it to you," the Democratic nominee said of the GOP. "On Tuesday, you can do it to them."

The afternoon rally here ended a day of critical campaigning during which Mondale repeatedly portrayed a second Reagan term as a threat to minorities.

From a church pulpit in Memphis, Mondale invoked the legacy of Martin Luther King Jr. to paint a bleak picture of a second Reagan term.

"We made more progress toward social justice and decency in these last years before Reagan than in any time and in any country in American history. Lose this election and are you hopeful? Are you going forward? Where's the progress?" Mondale told 500 morning worshippers at Monumental Baptist Church.

Then, in McAllen, Tex., near the Mexican border in the Rio Grande Valley, Mondale promised an overflow crowd that he would be the first president to put a Hispanic in the Cabinet.

With the last round of public opinion polls showing his party facing a possible rout in Tuesday's elections, Mondale ended the day here with promises to preserve local military installations that bolster the area's economy.

It was a rousing day for Mondale. The church choir in Memphis greeted him with rocking gospel music. In Corpus Christi, singer-guitarist Jose Feliciano warmed up the crowd with a version of his old hit song that asked, "C'mon, Mondale, light our fire . . . . Set the election afire."

At each stop in Texas, Mondale's 22-year-old son William boomed out a flowing introduction of his father -- in Spanish.

Democratic and Republican strategists expect Mondale to be the overwhelming favorite among both ethnic groups. The best hope for Mondale and many other Democratic candidates in any southern, southwestern or border state lies in record voter turnout among blacks and Hispanics. For the past few weeks, however, Mondale had spent little of his campaign time courting these groups.

In Memphis, there was a bountiful blend of church and politics. It began when the Rev. Samuel B. Kyles belittled Reagan's explanation of why he does not attend church services regularly.

"Mr. Reagan said he didn't go to church because it was too dangerous. We didn't know 'too dangerous' for who," Kyles said. "Maybe Mr. Reagan didn't want to go to church because the Holy Ghost might come on him and make him do right."

Mondale made no mention of the dangers of mixing religion and politics -- a major topic when he spoke to Jewish groups earlier in the week. Instead, he delivered an informal variation of his standard political stump speech and castigated Reagan's policies.

He blamed the administration for an increase in the number of children in poverty, saying that 4 million had been cut from the school lunch program and that, under Reagan, infant mortality is on the rise.

This administration, Mondale said, is "cruel, uncaring, brutal, unkind and vicious to the helpless in America and that's not our faith."

Reagan, he said, has "trashed" the U.S. Commission on Civil Rights. "Not even Nixon did that," Mondale said. "And if he didn't do something, it's got to be really bad."