The rising tide of conservatism threatening to end Sen. Frank Church's quarter-century in the Senate was visible here Tuesday evening at a Republican fund-raiser for Rep. Steve Symms in the comfortable south Boise home of lawyer Robert Koontz.

What made the cocktails and hors d'oeuvres more than routine was that Koontz and a dozen others among the several score Republican fat cats who paid $100 each to Symms' campaign had sat on their hands when Church swept to his fourth Senate election six years ago. Indeed, several actually sweetened Church's Domocratic campaign kitty in 1974 -- but not in 1980.

That is an indication on four-term Rep. Symms as a Republican who can finally end the Senate career of the chairman of the Foreign Relations Committee. It is also a measure of what one Republican here calls "the Reagan revolution" that has made big government, high taxes, welfare spending and weakness abroad heavy burdens for liberal Democrats in 1980.

The campaign between Church and Symms is the ideological litmus test of whether the "Reagan revolution" has enough force to defeat liberal Democratic senators from California to New Hampshire. Church is running scared. His latest poll shows him slightly ahead -- but under 50 percent, an unsafe level for an incumbent.

For Symms, an earthy right-winger and ex-Marine, gaining political credibility with business and banking sophisticates of Boise is a significant breakthrough. For Church, their enlistment in Symms' cause, despite the pretige that the Foreign Relations Committee chairman gives to Idaho, is a signal that his support is dangerously weakened. He claims credit for using his proximity to the world's mighty as a tool in the service of his state. Speaking to seed men from western Idaho in Twin Falls one day this week, Church told how he had arranged sales of Idaho's rich agricultural products to foreign countries.

But the value of such claims must compete with the senator's more public actions, particularly in the last 10 years. His vote in favor of the Panama Canal treaties hurt badly in nationalistic Idaho. Church has trimmed back on other issues as the Nov. 4 election closes in: he fought the B1 bomber, but two weeks ago suddenly voted to support the development of a new penetration bomber; he praised Cuba's Fidel Castro in 1977, but last year took it upon himself to make the first announcement of the Soviet combat brigade in Cuba. That embarrassed the Carter adminstration, but made Church look like Paul Revere.

The senator told us in Twin Falls that his 1977 compliments for Castro were simply carrots to persuade Castro to free Americans held in Cuba. But Church used the opposite tactic -- the stick of punishment by the United States -- when it came to trying to persuade Saudi Arabia to join the Camp David peace process. His first major speech last year after becoming Foreign Relations chairman was an attack on Saudi Arabia so extreme that The Washington Post took him to the woodshed for it.

Church, whose voluminous foreign travels have not once taken him to Saudi Arabia, told us the Castro and the Saudi affairs were in no way comparable. But some Republicans here think differently: that Church's whiplashing of Saudi Arabia was to mollify Israel's concern over the United States' becoming too chummy with its No. 1 oil supplier.

Middle East politics, in fact, is the largest hidden issue in this campaign. Symms is attacked by Church as a potential "senator from Exxon" because of Texas oil money moving into his campaign. The Symms camp, using reports filed with the Federal Election Commission, charges that Church is running almost entirely on contributions from "the liberal Eastern establishment" -- heavily larded with donations from the American Jewish community.

When actress Elizabeth Taylor canceled a promised appearance here for Symms in April with her husband, Sen. John Warner, the reason given was a schedule conflict; Republicans believe the real reason was that Liz Taylor, a convert to Judaism, was persuaded not to do anything that might hurt Church.

There has been no trimming by Church on Mideast politics. Elsewhere, the senator is skillfully adjusting to the conserative surge, cashing in 24 years of hard work for his constituents -- and praying that the strong Reagan tide here won't wash him out of office.

Because of a transmission error, the Wednesday column referred to 1,000 new Minutemen III missiles. It should have read 100 new Minutemen III missiles.