The Republican National Convention opened today with a direct appeal to traditional Democrats to reject Walter F. Mondale as "a born loser" and endorse President Reagan's "record of peace, prosperity and pride in America."

As Dallas baked under another day of searing heat, the first round of convention oratory drew a sharp contrast between what Senate Majority Leader Howard H. Baker Jr. (Tenn.) called "a team that has proven it can succeed and a team that has proven it can't."

"We have come a long way in four years -- from the shame of Tehran to the brave rescue of American students in Grenada," said the official keynoter, U.S. Treasurer Katherine D. Ortega. "We have come from the weak leadership of the Carter-Mondale administration to the strength of the Reagan-Bush administration. We have come a long way. And we are not going back."

In what was clearly the most emotional moment of a generally low-key evening, Jeane J. Kirkpatrick, a Democrat and the Reagan administration's U.N. ambassador, severed her remaining ties to the Democratic Party. She did so by linking Mondale, a longtime political ally, by name to "the dismal period of retreat and decline" in America's world position, her characterization of the Carter years.

Kirkpatrick, using tougher language than any Republican has applied to the opposition, tied Mondale to the "blame-America-first crowd" that she said has captured the Democratic Party and persuaded the party that "it could shut out the world by hiding its head in the sand."

Ortega, Kirkpatrick, Baker and Health and Human Services Secretary Margaret M. Heckler were the featured speakers in the bunting-draped Dallas Convention Center, where 2,235 delegates and 15,000 spectators shared a sense of optimism about the Nov. 6 general election that soared almost as high as the 100-degree-plus temperature outside.

Voters will reject "the failed policies of the past" and recognize the GOP as "the party of America's future," Republican National Chairman Frank J. Fahrenkopf Jr. said in opening the 33rd Republican convention this morning.

Fahrenkopf and other top GOP leaders have expressed fear that complacency could endanger the victory they foresee, and the delegates' cheers tonight seemed more dutiful than spontaneous. No such problem is expected when Reagan appears Thursday night to accept renomination.

Still, for those with a sense of history, the appearance of Kirkpatrick, a longtime friend and political comrade of the late Hubert H. Humphrey, on the podium of the Republican convention, and her condemnation of her party's current leadership and foreign policy constituted a landmark event.

On a day when the Democratic vice presidential nominee, Rep. Geraldine A. Ferraro (N.Y.), was trying to quiet reports of tax problems by making a full financial disclosure, the Republicans took a deliberately tolerant stance toward her while slamming Mondale in unrestrained fashion.

Vice President Bush, representing the ticket while the president campaigned in Illinois and Ohio, avoided commenting on Ferraro as he toured state delegation caucuses and praised Reagan.

"The American people see this president as a strong, principled leader, a man of compassion, a man of concern, but a man who's not afraid to make a decision," Bush told New York delegates.

Meanwhile, Sen. Paul Laxalt (Nev.), general chairman of the Reagan-Bush campaign and the Republican Party, told an afternoon news conference that Mondale, a former Senate colleague, is "a born loser."

Baker, temporary convention chairman and first of the 1988 presidential hopefuls to be showcased from the podium, tonight picked up on that theme when he said Mondale "can't stand" to see America's success under Reagan's leadership.

"So he's trying to invent another America," Baker said, "a trembling, despairing, miserable America that needs the Democratic Party to come to its rescue . . . . But it's not going to work. Walter Mondale is running for president, and he ain't no FDR."

Laxalt said Mondale deserves blame for Ferraro's problems and asked why Mondale did not "quickly seize control of this disclosure problem and quickly clean it up. I hope this woman survives," he said. "I hope it doesn't create a bad precedent for women in the future of politics."

The opening salvo of oratory confirmed the Republican strategy of focusing on "the leadership gap" between Reagan and Mondale while playing down specifics of Reagan's second-term agenda.

"The bottom line is leadership," Reagan-Bush campaign chairman Edward J. Rollins said. "The American public sees Ronald Reagan as a leader who has done what he set out to do. Mondale has an ill-defined image."

Orators on this opening day were trying to fix in voters' minds a picture of Mondale who was trained by President Jimmy Carter in what Fahrenkopf called "arguably the least effective presidency in history" and who demonstrated at the Democratic National Convention in San Francisco last month that he "is just a man who can't say no."

"He made so many promises to so many pressure groups that it's clear Mr. Mondale left more than his heart in San Francisco," Fahrenkopf said. It is no wonder, he said, that many Democrats are "embarrassed by their nominee, ashamed of their party and bewildered by their platform."

"Misery has become very important to Walter Mondale," Baker said early in the evening session. "When he's in office, he creates it. When he's out, he invents it . . . . Walter Mondale has nothing to offer a successful America."

Ortega, a previously obscure official named to the prestigious keynoter post, said she believes in Reagan "not because I am a woman and not because I am of Hispanic heritage" but because he is "a great leader" who restored to America "respect lost when President Carter was in the White House, Walter Mondale was his vice president and 52 American citizens were held hostage by a mob for 444 days."

Ortega added, "Let the Democrats run their campaign on rhetoric. We Republicans will run ours on the Reagan record of peace, prosperity and pride in America."

Ortega quoted one of Mondale's former rivals for the Democratic presidential nomination, Sen. Gary Hart (Colo.), as saying Mondale offered nothing but "a collection of old and tired ideas, held together by paralyzing commitments to special interests and constituency groups. "

"Now I don't ordinarily agree with Gary Hart," she said, "but in this case, as treasurer of the United States, I can certify that he was right on the money."

She said an example of Mondale's "old, tired ideas" is his call for higher taxes and predicted that "millions of mainstream Democrats" will support Reagan rather than endorse "high taxes, big spending and more government regulation."

To those Democrats, she said: "We Republicans welcome you to our home. Nuestra casa es su casa. Our home is your home."

Heckler argued that when Mondale contends that "raising taxes will make things better," he proves that "there are none so wrong as Walter Mondale, who will not learn."

Heckler, a former member of Congress from Massachusetts, made her appeal to Democrats by recalling the question that House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) asked her when she was elected: "Why of all things is Margaret Mary Catherine O'Shaughnessy Heckler a Republican?"

The answer, Heckler said, is that "the Democratic Party delivers rhetoric; the Republican Party delivers results."

Rollins, in a meeting with reporters, made it plain that Reagan's advisers have decided for now to emphasize claimed successes of the president's first term and the perceived weakness of the opposition rather than to focus on Reagan's second-term agenda.

While Laxalt spoke at a news conference of the Republicans' "visionary program for the future," Rollins said that "you're not going to see a lot of program initiatives and promises out there. Mondale has made enough promises, and we're ready to debate them. The president will run on his record and leadership."

Earlier this summer, Rollins was identified with those on the White House staff and in Congress who argued that Reagan should be specific in setting forth second-term plans, particularly on economic policy, in order to make reelection as much of a mandate as possible. But with GOP and independent polls showing Reagan 13 to 15 points ahead of Mondale, the counsel of caution has prevailed.

"The mandate comes from the voters," Rollins said, "and once you have it, you can move forward any kind of program."

Rollins predicted that there will be two Reagan-Mondale debates and a single Bush-Ferraro confrontation.

But he, like other senior campaign and administration officials, was careful to say Reagan would not necessarily be bound by specific platform pledges barring tax increases and calling for further tax cuts.

"Many things become administration policy that are not in the platform," Rollins said, "and many things in the platform never become administration policy."

Treasury Secretary Donald T. Regan underlined that message when reporters asked about a platform promise to raise the personal income tax exemption from $1,000 to at least $2,000 and he replied, "We don't want to box ourselves into a particular tax system."

The platform is theoretically subject to amendment when it comes to the floor Tuesday, but state caucuses today found overwhelming support for the conservative document and little disposition to change it.

The Platform Committee approved the manifesto 76 to 3. Sen. Lowell P. Weicker Jr. (R-Conn.), leader of the outnumbered liberal dissidents, conceded, "The votes just aren't there to change it."

Speakers on today's program vied with each other in finding ways to praise the platform and the party as embodiments of a better future for all Americans, particularly those with historic links to the Democratic Party, including blacks, members of ethnic groups and Hispanics.

The Republicans showed off one of their newest recruits, former professional football star Roosevelt (Rosey) Grier, long associated with the late Robert F. Kennedy and the Kennedy family.

Grier said he learned when he went to Washington this year to lobby for "President Reagan's initiative for voluntary school prayer . . . that many of the people I supported in the past were on the other side . . . . So I'm standing behind President Reagan 100 percent."

Grier, in turn, introduced to the audience several American athletes who won gold medals at the recent Summer Olympics in Los Angeles.

Meanwhile, Reagan campaign leaders increased pressure on the major television networks to carry the 18-minute film introducing Reagan's acceptance speech Thursday night.

In separate meetings with network executives, they argued that it would balance the 20-minute convention introduction of Mondale by Sen. Edward M. Kennedy (D-Mass.) last month.

The networks have made no commitment, officials said.