BOISE, IDAHO -- For Democrat Larry LaRocco, a 44-year-old stockbroker who is making his second bid to become a congressman, the road to the House of Representatives may lead through the recruiting department of the National Republican Senatorial Committee.

Eight years ago, after losing a challenge to first-term Republican Rep. Larry E. Craig in a recession year that was favorable to Democrats, LaRocco swore off running against incumbents and their many advantages. The odds were just too long in a district that elected its last Democrat in 1964, the year that Lyndon B. Johnson's landslide helped Democrats pick up 37 House seats.

But this year, LaRocco is back on the campaign trail. Unlike 1964, it is not a strong national Democratic ticket, but Republican good fortune that has enhanced his prospects.

LaRocco's opportunity stems from incumbent Craig's decision to seek the Senate seat of Idaho's retiring senior Republican senator, James A. McClure. While Craig's candidacy has given the GOP a good chance of retaining McClure's Senate seat, it has left the party open to a vigorous challenge in what has been regarded -- despite its history of electing Republicans -- as the more Democratic-leaning of Idaho's two congressional districts.

Nor is Idaho an isolated example of this off-year phenomenon. From Hawaii to Rhode Island to Arkansas, GOP success in recruiting House members to run for the Senate and for governor has left Republicans struggling to hold onto seats they have been able to take for granted in recent years. It has also lengthened the odds against the Republicans whittling down the Democrats' 82-seat margin in the House.

Even some Republicans concede that the Democrats have the advantage in this year's battle for the 30 House seats left open by retirement and ambition. Said Rep. Guy Vander Jagt (R-Mich.), chairman of the National Republican Congressional Committee: "We are defending 18 open seats, and they are defending 12. A number of our best people are running for the Senate, so going in we were bucking the odds."

Yet even Democrats are a bit surprised at the extent of their good fortune. They expected to be competitive in such Democratic strongholds as Hawaii, where Republican Rep. Patricia Saiki's Senate bid has opened up her House seat, or Connecticut, where former representative Toby Moffett (D) hopes to take the district left open by Republican Rep. John G. Rowland's run for governor.

But this year, Democratic House candidates are also running well in some unlikely locales. In Iowa, Colorado and Nebraska, for example, Democratic candidates are proving to be genuine contenders for House seats being vacated by Republican Reps. Thomas J. Tauke, who is running for an Iowa Senate seat, Hank Brown, running for the Senate in Colorado, and Virginia Smith, who is retiring.

Republicans are also plowing the open-seat field, which in a time of 98 percent incumbent reelection rates usually provides the best opportunity for either party to make gains. The GOP has fair prospects of winning seats now held by Democratic Reps. Marvin Leath of Texas, who is retiring, Joseph E. Brennan of Maine, who is running for governor, and Bill Nelson of Florida, who lost a bid for the Democratic gubernatorial nomination. But with only a month to go before the Nov. 6 election, Democrats seem poised for a richer harvest of open-seat victories.

"That's where we make our net gains," predicted Rep. Beryl Anthony Jr. (D-Ark.), chairman of the Democratic Congressional Campaign Committee.

Idaho's 1st Congressional District, which stretches 500 miles from the mountainous border with Canada to the desert abutting Nevada, demonstrates how an incumbent's decision to relinquish a House seat can transform a safe district into a competitive one.

The Idaho GOP has fielded a strong candidate to succeed Craig, Skip Smyser, a 10-year veteran of the Idaho state Senate. But Smyser's home base in the Republican stronghold of Canyon County casts only about one-fifth of the district's vote, and the Parma resident has struggled to raise his profile.

But Smyser believes that a long-standing controversy over Idaho's millions of acres of wilderness -- how much should be preserved, how much should be open to logging and other uses -- will help crack the traditionally Democratic panhandle in the north, where the union influence of the timber and mining industries is still strong.

Smyser accuses LaRocco of wanting to protect so many acres of wilderness that it would devastate the forest products industry, and he repeatedly reminds voters that the Democrat has been endorsed by the Sierra Club.

In return, LaRocco points to his time as the northern Idaho aide to the late senator Frank Church (D) and his success at negotiating nettlesome wilderness disputes among warring parties.

But LaRocco believes his trump card is abortion, an issue that could overturn the GOP's registration advantage in populous Ada County, which includes the state's largest city and its capital, Boise, and provides about 28 percent of the district vote.

LaRocco casts his support for abortion rights in terms of individual liberty and government interference. "Idahoans are fiercely independent," he says. "They don't like government telling them what to do."

But even some LaRocco supporters say that abortion does not have the resonance it did earlier this year when an abortion battle in the state legislature captured national attention. Moreover, LaRocco has left himself open to criticism by Smyser that he has changed his abortion position to fit current fashion. Though LaRocco now says he was only expressing his personal opinion, in his 1982 House race and a 1986 state Senate contest he favored abortion only in narrow circumstances.

"It's very obvious my opponent has changed his position because the district is now pro-choice," said Smyser, who favors abortion only in cases of incest, rape and to protect the life of the mother.

Both sides may differ on how the wilderness and abortion issues will play here, but they agree on one thing: The race in Idaho's 1st District is likely to be more competitive than it has been in many years.

Bethine Church, the widow of the senator who knows a little about shifting political moods after her husband's stunning defeat by Sen. Steve Symms (R) in 1980, says simply: "I do have this feeling that there's more of a Democratic tide running this year."