The temperature is extremely cold in Iowa, but the rhetoric is plenty hot as the Democratic presidential candidates compete for support in the upcoming caucuses.

About 100,000 Democrats are expected to show up for the Jan. 19 caucus. That's the same number of members that belong to both the Teamsters (search) and United Auto Workers (search) unions, two of the three labor groups driving the grassroots effort in Iowa. The third union is the American Federation of State, County and Municipal Employees, which has about 30,000 members.

The first two groups support Missouri Rep. Dick Gephardt. AFSCME (search) backs former Vermont Gov. Howard Dean.

Besides running television ads, hosting candidate forums and providing all kinds of manpower, the unions are under considerable pressure to turn out their members to vote.

"If all of Gephardt's folks were to go out, if all of Dean's folks were to go out, Gephardt would have a significant advantage," said Kay Henderson of Radio Iowa.

Whether those folks show up or not is probably the biggest pressure point for the candidates.

Click here to watch a fair and balanced report from Fox News' Steve Brown.