(CNN) -- Sens. Hillary Clinton and Barack Obama have aggressively courted organized labor, but unions are divided between the Democratic candidates.
Sen. Hillary Clinton talks to workers in Portage, Indiana, on Wednesday.
Clinton and Obama frequently address issues that hit home for unions -- wages, protection of the right to form unions, health insurance, cutting taxes for the middle class and reworking trade agreements that some union members blame for job loss.
A union's endorsement can give a candidate's campaign a significant boost because union members often act as ground troops to canvass neighborhoods and work the phones.
Speaking in Portage, Indiana, Clinton on Wednesday told workers "the American labor movement built the middle class."
"I will fight with you and for you. And no state needs a president more who understands the importance of manufacturing and the significance of the labor union movement than Indiana," she said in her speech at Duneland Falls Steel Workers Local Union Hall.
Obama last month told union members in Pennsylvania he's "ready to play offense for organized labor."
"It's time we had a president who didn't choke saying the word 'union.' It's time we had a Democratic nominee who doesn't just talk about unions in the primary," he said.
"We need a president who knows it's the Department of Labor and not the Department of Management. A president who strengthens our unions by letting them do what they do best -- organize our workers." Watch how the candidates are campaigning in the Midwest »
Clinton has the support of many public sector unions, including the American Federation of State, County and Municipal Employees Union, the American Federation of Teachers and the United Farm Workers.
Obama has the support of the Service Employees International Union, Teamsters and Unite Here, which represents hotel and restaurant workers.
Unite Here's leader, Bruce Raynor, said he does not trust Clinton when she says she will rework the North American Free Trade Agreement.
"We have a big problem with believing that the Clintons are committed to free trade policies that would protect American jobs. That worries us," he said. "Sen. Obama, on the other hand, has been with us from day one."
Clinton has faced skepticism because her husband, former President Bill Clinton, supports a free trade agreement with Colombia. Clinton insists she staunchly opposes it.
"I don't think any married couple I know agrees on everything, and we disagree on this," she said last month.
The AFL-CIO, the umbrella group for many major unions, has not endorsed a candidate, but the group has launched an aggressive attack against McCain.
The $53 million effort, called "McCain Revealed," aims to educate voters on the Arizona Republican senator's record, which the labor group says has been consistently anti-working families.
A Republican National Committee spokesman called on Obama and Clinton to denounce the AFL-CIO's efforts, saying it would be consistent with both senators' denunciations of special interest groups.
"The AFL-CIO's campaign against John McCain clearly demonstrates their priorities lie in attack politics as opposed to focusing on American families," RNC spokesman Alex Conant said.
"Voters looking for something new will find it in John McCain's campaign to help working families -- not the AFL-CIO's partisan attacks." E-mail to a friend
CNN's Allan Chernoff contributed to this report.