The mission of Ronald Reagan, says his wife Nancy, "is to revive the Republican Party." Or, in the view of moderate Republican leaders, to bury it. Actually, the party is already teetering on the edge of the grave, if numerous new post-mortems are to be believed.

Almost simultaneous with Mrs. Reagan's vision has come a Gallup report disclosing that the GOP has declined to the lowest point in the history of the poll, which says only one American in five now classifies himself as a Republican.

That findings is confirmed in the latest issue of Fortune, which features the first of three articles on "The Unmaking of the Republican Party," written by Everett Carll Ladd Jr., a political scientist at the University of Connecticut. Ladd says defections from the GOP have been so severe that it no longer is seriously competitive with the Democrats. It is weaker now, he adds, than even the Democrats were around 1900.

Former ambassador Henry Catto Jr., a prominent young Republican, thinks "a comeback may occur," but, he cautions, "one is increasingly reminded of a dropped rubber ball: Each succeeding bounce is less high than the preceding one until there is no motion at all."

Nevertheless, the scholarly Fortune study concludes that, "for all its weakness, the GOP does not appear about to go the way of the Federalists and the Whigs." It realistically notes that the electoral laws tend to retard the development of new parties, and that three appears to be no "hook" on which to hang a new major party. The prospect, it concludes, is that the United States "will continue to limp along with the confusion and diminished competitiveness of its new one-and-a-half party system."

Meanwhile, the Ford with of the party can hardly be blamed for doubting that the best way to launch a comeback is by all-out partisan opposition to the Panama Canal treaties, a crusade led by Reagan and his right-wing senatorial followers. One distinguished GOP elder privately warns that it is a risky hook "on which to hang the future of the party."

It is an issue that splits the party right down the middle, as was demonstrated in the bitter Ford-Reagan presidential primary battles last year. Moreover, it is so volatile that it could explode in the party's face, for defeate of the treaties is likely to trigger violent protest in Panama and possible sabotage of the canal.

The opposition's preoccupation with the canal seems to have blinded it to the vulnerability of the Democratic administration on the Bert Lance case. Although it is the best opening the Democrats have given the Republicans in years, the opposition leaders have done relatively little to exploit it.

Until a week or so ago, in fact, Republicans on the Senate Governmental Affairs Committee, charged with reviewing the allegations against Lance, were as zealous as the Democrats in defending the budget director.

Sen. Charles McC. Mathias Jr. (R-Md.) is one of the most respected men on Capitol Hill in either party. The other day he was asked how his party's base should be broadened. His answer was: "First, you stop inviting people out" - meaning the determination of the currently dominant right wing to eject the moderates.

When Mathias was asked whether moderates and liberals have any future in the Republican Party, he said, "I think the question should be rephrased: "Will the Republican Party have any future without a broad-based membership that includes moderates and liberals? That kind of party is the only one that can recover vigor and assert leadership."

In looking to 1980, however, another prominent Republican, John Connally of Texas, who is not without White House aspirations of his own, predicts that "you're going to see very few, if any, new faces rise to the surface in the presidential primaries." He says they "are going to be the old faces - they always are. I don't think anybody who's not now something of a political figure has a chance to be a factor in 1980."

Maybe, but that's not the view of one of the brightest members of Congress, Rep. John B. Anderson of Illinois, chairman of the House Republican Conference. He says: "We will develop a new leader between now and 1980. I'm convinced of that."

He sees no chance that Ford (now 64) or Reagan (now 66) will be nominated three years from now. He also disagrees with the prevailing notion that only an outright conservative has a chance of taking over the reins of the party in 1980. He thinks the GOP could do worse than nominate the governor of his own state, Big Jim Thompson, or the governor of Michigan, WIlliam Milliken. Indeed, the party could do worse than nominate Anderson himself.

In all probability, the Republicans will have to run against an incumbent President in 1980. Since that is usually a hopeless task, it might be a good time to gamble. Catto's long-shot idea is for the GOP to nominate a woman.

A serious move for the presidency by an able and experienced woman, he believes, "would underline the rights issue and outflank the Democrats. It would bring forth a gusher of new volunteers. It would give life to a generally staid party." Could be.