from CNN headlines: CNN) -- Sen. Barack Obama received the backing of a major service workers union Friday, a significant boost to the Illinois Democrat's campaign that comes amid signs that his rival's support among superdelegates may be slipping. Sen. Barack Obama campaigns in Waukesha, Wisconsin, on Wednesday. "We have an enormous amount of respect for Senator Clinton, but it's now become clear members and leaders want to become part of an effort to elect Barack Obama the next president," SEIU president Andy Stern said during a conference call announcing the union's endorsement. Three sources told CNN that union leaders had deliberated on the endorsement via a conference call Thursday. Obama was the overwhelming choice of the union's state and national leadership, they said. Obama also received the endorsement of the 1.3-million member United Food and Commercial Workers Union on Thursday afternoon. A union's endorsement can give a candidate much needed support because union members often act as "ground troops" that can canvas neighborhoods and man phone banks for a campaign. The two union endorsements could also help Obama in his increasingly heated struggle with Sen. Hillary Clinton for the Democratic presidential nomination, especially among blue-collar voters in the delegate-rich states of Ohio, Texas and Pennsylvania. Watch Clinton battle for Ohio » Clinton's own supporters suggest that the New York senator must do well in Texas and Ohio primaries on March 4 if she is to stop the momentum Obama has built by winning eight states in a row. The Clinton campaign has also said it is looking for a strong showing in the Pennsylvania primary on April 22. Obama now leads Clinton in the overall delegate count -- 1,253 to 1,211, according to CNN calculations. News of the union endorsements comes as a superdelegate -- oneof the Democratic Party officials or elected officials who could decide the nomination at the party's convention in Denver, Colorado, this summer -- said he would vote for Obama instead of Clinton, as he had previously pledged to do. Rep. David Scott, an African-American from Georgia, told the Associated Press he would vote for Obama because he did not want to go against the will of the voters. Obama won the Georgia primary on Super Tuesday, February 5, and 80 percent of Scott's district voted for him, the AP reported. "You've got to represent the wishes of your constituency," Scott told the AP on Wednesday. "My proper position would be to vote the wishes of my constituents." The New York Times reported Friday that another black lawmaker from Georgia, Rep. John Lewis, was also going to shift his support to Obama from Clinton. Lewis is one of the most senior African-American members of Congress and a respected voice on civil rights. Lewis' office, however, told CNN that The Times misrepresented his intentions and said Lewis had not decided to switch his support to Obama. But the AP reported that many sources close to the Georgia lawmaker said he was torn over his earlier endorsement of Clinton. Clinton has sharpened her attacks on Obama in recent days, possibly in response to the do-or-die situation she is facing over the next couple of weeks. While campaigning in Ohio on Thursday, Clinton accused Obama of caving in to special interests like the nuclear and oil industries and questioned her rival's ability to deliver on his rhetoric. "There's a big difference between us -- speeches versus solutions, talk versus action. ... Speeches don't put food on the table. Speeches don't fill up your tank or fill your prescription or do anything about that stack of bills that keeps you up at night," she said Thursday during a campaign stop in Youngstown, Ohio. Watch Clinton go after Obama » Reacting to Clinton's charges, Obama spokesman Bill Burton Thursday said his candidate "doesn't need any lectures on special interests from the candidate who's taken more money from Washington lobbyists than any Republican running for president." "Sen. Clinton may have said that attacks and distortions are the 'fun' and 'exciting' part of the campaign, but they're exactly what everyone else in America is tired of," Burton said.