DES MOINES -- Although many continue to doubt his electability, Sen. Paul Simon (D-Ill.) has been chipping away at support for Rep. Richard A. Gephardt (D-Mo.) in organized labor, one of the most important constituencies in the Iowa Democratic caucuses.

Gephardt retains a decided edge among this state's union leaders, particularly the United Automobile Workers, according to strategists in most campaigns. But Simon, a late starter in the Democratic contest, has picked up the personal endorsements of a number of union chiefs -- not only in Iowa cities near the Illinois border, but also from machinists and building trades officials here in the center of the state.

Chuck Gifford, head of the Iowa UAW and one of Gephardt's strongest backers, said that Gephardt -- the sponsor of trade legislation empowering the president to restrict imports from foreign competitors -- remains the overwhelming favorite of UAW leaders and rank-and-file, "but in terms of auto {UAW} in this state, it's a two-candidate race" that is limited to Gephardt and Simon.

In large part because of his sponsorship of the trade legislation and his three-year courtship of labor leaders here, Gephardt has been widely viewed as virtually holding a lock on Iowa unions, which can be critically important sources of support in a caucus state where strong turnout from disciplined supporters can make the difference between victory and defeat.

"We put a lot into these caucuses," said Jim Wengert, president of the Iowa Federation of Labor (AFL-CIO) and neutral in the presidential fight. "We run precinct caucus schools for our members," at which they learn in advance exactly how the local gatherings work. Wengert said turnout from unions and the Iowa Education Association amounted to slightly more than a quarter of the total participants in past presidential-year caucuses.

"These caucuses are tailor-made for an organization like ours," the UAW's Gifford said. "Our people understand this caucus process."

Simon's ability to compete with Gephardt among leaders of organized labor has been demonstrated by his showing in a number of straw polls at union gatherings. At both a UAW meeting Saturday morning and a union-backed "Jobs for Justice" rally that afternoon, a significant number of union members wore Simon buttons -- including Ray Sullivan, head of the Des Moines Building Trades, and Steven Piper, president of the Municipal Laborers Local 353.

At both union and other meetings here over the past weekend, however, Simon, who claims to be an outspoken liberal in the tradition of Harry S Truman, raised eyebrows among prospective supporters because of some of the vagueness in his speeches.

In a speech to UAW leaders from Iowa, Illinois and Nebraska, Simon declared, "We can have a much healthier rural America if we have a government that is sensitive." A listening UAW official leaned over and asked an associate in a whisper: "Now what the hell does that mean?"

Later that day, Simon told a group of former supporters of Sen. Joseph R. Biden Jr. (D-Del.) that "we ought to be demanding of other countries that they pay their workers more." One of those attending the meeting, Michael Tramontina, a former executive director of the Iowa Democratic Party, countered that a policy of asking foreign countries to raise workers' salaries is sure to fail.

Thomas Whitney, former state Democratic chairman and an admirer of Simon, said after watching the Illinois senator perform that he has been struck by Simon's failure to gain the combination of "credibility and power" that should naturally accrue to an announced presidential candidate who holds statewide office in such a major state as Illinois.

"There is the sense or feeling that he is not going to be there in the end," Whitney said.

For both Simon and Gephardt, the competition for support from organized labor takes place at a time when unions here are taking a severe beating. In the last decade, the number of UAW members has dropped from 157,000 to 67,000, Gifford said, noting that "10 years ago, we had wall-to-wall employment."

Wengert said membership in the unions in the state federation of the AFL-CIO has dropped from 75,000 to 50,000 in the last decade.