The night before the Iowa poll revealed a catastrophic 24-point drop in Ronald Reagan's strength, the possibility that next Monday's caucuses could devastate his 12-year search for the Republican presidential nomination was raised by a vivid contrast here in east cental Iowa.

Principal speaker for a party fund-raiser was George Bush. Sen. Howard Baker's daughter Cissy was present. So were campaign leaflets and posters for Bush, Baker, Connally, Crane and Dole. But there was no sign for Reagan -- not a campaign button, not a poster, not a word -- in Reagan's rural heartland.

"This is what we've been complaining about for months," a local Reagan loyalist told us. "There is no visible Reagan campaign." That absence, when compared with Bush's painstakingly built organization, suggests Reagan could finish second to Bush at Republican precinct caucuses Jan. 21. Reagan's overwhelming grassroots popularity, counted on to negate Bush's organizational edge, has been undercut by the infrequency of his visits to Iowa, climaxed by his boycott of the Jan. 4 presidential debate.

Since Reagan's campaign is based on invincibility and inevitability, losing Iowa would not be easily overcome. Thus, the cautious strategy of campaign manager John Sears undergoes an early critical test. If "Dutch" Reagan loses the state of his young manhood, the Sears strategy will be blamed.

That chorus would be led by Lt. Gov. Terry Branstad, 33-year-old hope of Iowa conservatives and a 1976 Reagan activist. Branstad repearedly has warned Reagan headquarters of Bush on the march in Iowa. When he spotted Sears in the Fort Des Moines Hotel coffee shop the other morning, Branstad complained that Sears had not returned his calls for months (the debonair Sears replied with a smile and his patented chuckle).

Branstad did get five minutes alone with Reagan himself during a rare Iowa stop Dec. 14, and gave him detailed advice: come to Iowa often, not for one monster rally but to hit several towns; above all, participate in the debate. None of this advice was followed. Branstad now gives Bush at least an even chance to win.

Senior Reagan strategists privately admit they underestimated Bush. Although they denigrate Bush's establishment backing as "the charity ball set," they admit he has locked up key local leadership. Although Bush at his best cannot match Regan's charisma on an off-day, he will have spent 27 days in Iowa by caucus time; when Reagan comes to Iowa Saturday, it will be his sixth day.

Reagan's campaign did not reach full gear until December, when Kenny Klinge, a skilled professional from Virginia, was dispatched to Des Moines. His phone bank has found over 50,000 Iowa voters "favorable" to Reagan. Klinge feels that Bush's operation is bogged down with the 25,000 republicans who attended the 1976 caucuses, while Reagan seeks a new blood.

Iowans contend Klinge is running a primary election campaign unable to dragoon ordinary citizens to play politician a couple of hours on a cold winter night. While Klinge hopes for 60,000 to 100,000 at the caucuses, the consensus is a maximum of 40,000 -- the fewer the better, from Bush's standpoint.

But Reagan's hope in larger numbers is undermined by his refusal to debate. Steve Fausel, a young manufacturer attending Bush's rally in Burlinton, told us he worries about Reagan's age and "electability" but is more familiar and comfortable with Reagan's issue positions than with Bush's. Did Reagan's no-show at the debate bother him? "If he had debated," Fausel told us evely, "I wouldn't be here tonight."

After hearing a Bush speech no less conservative than Reagan in attacking big government and supporting hard-line foreign policy, Fausel seemed inclined to Bush. Although Bush's Iowa backers come mostly from the party's liberal wing, his only real quarrel with Reagan is experience. He told the Janesville dinner that as CIA director, he was approving national intelligence estimates, "not lecturing about it out there on the free-enterprise circuit" -- the closest he comes to outright criticism of Reagan.

Bush worries that the anti-Reagan vote will be diluted by Baker, rising with a lavish television campaign. But Bush technicians believe Baker's feeble organization cannot bring his supporters to market; they think Baker, even with Bush in the Iowa poll, could finish fourth or even fifth in the caucuses.

The second place in Iowa for Bush would help make him Reagan's top challenger, but leave him the formidable task of actually beating Reagan somewhere else. In contrast, a Bush win here would change the world. "Just in case we lose," one Reaganite told us, "I'm glad John [Sears] will be in Des Moines caucus night to explain why it doesn't mean anything." In that event, he would also be called on to justify his famous strategy.