PHILADELPHIA -- Gerald McEntee, the fiery president of the American Federation of State, County and Municipal Employees, welcomed Sen. Hillary Rodham Clinton here on Tuesday with a rousing introduction.

The next morning, in the same hotel ballroom, Henry Nicholas, president of Hospital Workers Local 1999, a large AFSCME affiliate, split with his national leadership as he offered an introduction and an endorsement for her rival, Sen. Barack Obama. Both candidates' speeches drew sustained applause from the hundreds of delegates gathered for the state AFL-CIO convention.

The back-to-back appearances highlighted a distinct split within the state and national union movements over the Democratic presidential primary. At the same time, however, the cordial receptions for each of the senators, along with interviews with numerous labor leaders gathered in Philadelphia, suggested that Pennsylvania union forces will have little trouble coming together to back whoever ends up winning the roll call in Denver.

"What I saw this week was absolutely terrific,'' said Wendell Young IV, president of local 1776 of the United Food and Commercial Workers Union, which has endorsed Mr. Obama.Blue collar workers in general, and rank-and-file union members in particular have proved welcoming constituencies for Mrs. Clinton throughout the primary season, one of the reasons the state is generally viewed as fertile ground for her candidacy. Just over 15 percent of Pennsylvania's wage and salaried workers were members of a labor union in 2007, according to the U.S. Bureau of Labor Statistics -- well above the national level of 12.1.

While the proportion of unionized workers in the U.S. has followed a general downward trend in recent decades, the number of unionized workers rose both in Pennsylvania and the nation last year. In 2007, the number of workers belonging to a union rose by 85,000 to 830,000 in the state.

Exit polls showed Mr. Obama winning big among union households in Wisconsin, but that was an exception. In Ohio, Mrs. Clinton won the union vote easily. In California, she also carried union households by double-digits. The latest Franklin and Marshall College survey, released in mid-March, showed the New York senator with another big early lead among voters from union households in Pennsylvania.

Both Democrats head toward the state's April 22 primary with substantial labor allies. Chief among Mrs. Clinton's labor supporters is AFSCME, the state's largest union. The union's trademark green T-shirts dotted the hotel ballroom as Mrs. Clinton poke.

The State Employees International Union and the United Food and Commercial Workers, are backing Mr. Obama.

The American Federation of Teachers, a significant force particularly in Pittsburgh and Philadelphia, is behind Mrs. Clinton. UNITE-HERE, whose members range from hotel workers to apparel workers, is in Mr. Obama's column as is the International Brotherhood of Teamsters, whose president, James Hoffa, will be in Pittsburgh campaigning for the Illinois senator later this week.

One of the state's largest unions, the United Steelworkers, is on the sidelines for the primary. The USW endorsed former North Carolina Sen. John Edwards at a Labor Day rally in Pittsburgh last year. It has remained neutral since he dropped from the race.

Ken Nagel, a member of the International Brotherhood of Electrical Workers and an Obama volunteer, was in the Illinois senator's Mt. Lebanon headquarters yesterday, listening to a pep talk from Sen. Dick Durbin, D-Ill., to a group of door-to-door canvassers. Of the fellow union members he was trying to woo to the Obama fold, he said, "They are engaged [by the election], but they're pretty divided.

"A month ago, people were asking me if he was a Muslim. Now they bring up Rev. Wright, but I think the momentum is on Obama's side."

After listening to the contenders at the Philadelphia convention, Edward Needham, a Sheet Metal Workers Union member from Bucks County, said, "I was impressed with both of them. Labor is going to come together whoever the nominee is, because people know it's survival.''

Richard Bloomingdale, the secretary-treasurer of the state AFL-CIO said of the convention: "The message we wanted everyone to leave with was that both of these candidates are good for us -- certainly better than John McCain.''

Despite that message from their leadership, it seems certain that many AFL-CIO members in the state will back Mr. McCain in the fall. Mr. Bloomindale noted roughly a quarter of the state's AFL-CIO members are registered Republicans.

Jack Shea, president of the Allegheny County Labor Council, said that the intensity of the primary campaign would leave some bruises.

"We're going to have to work to heal some of feeling that are going to be left over with out membership,'' he said, while endorsing his colleague's assertions that unity in November was a veritable certainty.

Mr. Shea's ACLC, like the state and national AFL-CIO, has yet to endorse in this race..

"They have to have two-thirds support, and neither one can get over that bar," he said.

One obvious reason why both candidates have substantial -- but not dominant -- labor support, is that their records and platforms on labor issues are so similar.

Both promise to strengthen the National Labor Relations Board, make its easier for union to organize work forces, and review and reform trade deals such as the North American Free Trade Agreement.

NAFTA, which was largely negotiated during the tenure of President George H.W. Bush and approved in Congress with the strenuous backing of former President Bill Clinton, has served as a point of attack and source of vulnerability for both campaigns.

The senators' current stands on the issue are virtually identical. Both say they would call Mexico and Canada back to the table to renegotiate the accord to bolster its labor and environmental provisions. But Mr. Obama has repeatedly assailed his opponent over the fact that the accord was approved on her husband's watch. Before the Ohio primary in early March, he used a robust television and direct mail effort to spotlight her husband's central role in its adoption.

Mrs. Clinton has said that she privately opposed NAFTA but supported it in public out of loyalty to her husband's administration. Mr. McEntee endorsed that version of the pact's history this week, saying that he had witnessed her opposition at the time.

Others from that administration, however, have said that insofar as Mrs. Clinton had reservations about the pact, they involved tactics and legislative timing, rather than the substance of the agreement. Preoccupied with the health care proposal that would eventually founder, she is described as arguing at the time that the administration should have been spending its political capital on that issue rather than trade.

Mr. Obama's efforts to make Mrs. Clinton pay a political price for her association with the pact were blunted by the reports that his chief economic adviser had quietly assured Canadian officials that some of the senator's criticism were tailored to his political audience. The Obama campaign denied that claim, but its ham-handed effort to deal with the controversy was a debilitating distraction from the effort to club Mrs. Clinton with the issue.

Last week, however, Mrs. Clinton faced her own staff-driven embarrassment over trade. The Wall Street Journal reported Friday that her chief strategist, Mark Penn, had met with officials of the Colombian government earlier in the week to advise them on how to win approval of a free trade pact now before Congress. Both Mrs. Clinton and Mr. Obama had assured the AFL-CIO members that they opposed the Colombian trade deal because of a pattern of violence toward labor organizers.

Mr. Penn, who said he met the South American officials in his non-campaign role as president of a major public relations firm, quickly issued a statement of apology, calling the meeting "an error in judgment that will not be repeated."

At this stage of their Pennsylvania drive, the Obama campaign has been notably less aggressive than in Ohio in attacking Mrs. Clinton on NAFTA and trade. Whether the Penn incident will give them a renewed opportunity to leverage the issue remains to be seen.

The upper reaches of the national labor movement for years, have been riven with disagreements on how best to advance their political agenda. In 2005, a coalition known as Change to Win, including the SEIU, UNITE HERE, the UFCW and the Teamsters, split from the leadership of the national AFL-CIO. Those overall disagreements remain, but union officials interviewed this week were unanimous in contending that they would not impede labor efforts on behalf of the Democratic nominee in the fall.

Still, those expressions of unity this past week in Philadelphia were coming from union officials. Beyond unity among its hierarchy, the union movement's greater challenge will to convince their rank-and-file members of the merits of the Democratic argument in the fall. Union groups on both sides of the Clinton-Obama divide were active in the voter registration efforts that produced a record Democratic registration total before the recent registration deadline. In the next two weeks, the union officials forecast that their volunteers would be manning phone banks and contacting members at work places and in neighborhood door-knocking.

Speaking of the officials who listened to the candidates in Philadelphia this week, state AFL-CIO president Bill George said, "The crowd going in here was one-third Hillary Clinton, one-third Obama, and one-third undecided. Where we had that one-third undecided, that might have lessened a little bit but I think its still about even.''

By virtue of his union office, Mr. George is a superdelegate and professes to be undecided in the Democratic race. Asked when he expected to make a choice, he offered this tongue-in-cheek fantasy, starring himself, for the Democratic convention in Denver.

"It'll be all tied up and there'll be one person that breaks the tie. And after all the money that's been spent and all the travel time and all the media, some steelworker from Aliquippa is going to walk up to the microphone and tell America and the world who the next president is."