With the presidential race closer than many expected, labor leaders here at the Democratic convention asserted themselves anew this week and cast their unions as saviors for a candidate who they say needs them more than ever. Despite Obama’s initial success without labor backing, labor leaders said no one is better poised to address what has emerged as his biggest challenge: winning over white working-class voters in key Rust Belt states where unions still maintain a considerable presence. Polls show Obama faring about as well with these voters as Sen. John F. Kerry (Mass.) in 2004, but to win in Pennsylvania, Ohio and Michigan, Obama probably needs to improve on that.

Labor leaders point to recent elections for proof of their impact. Even in a time of declining membership, voters living in union households turn out at higher levels and are far more likely to vote Democratic. In 2004, members of union households made up 37 percent of the vote in Michigan, 30 percent in Pennsylvania and 34 percent in Ohio, well above levels of union membership in those states. Kerry performed about 30 percentage points better in those states among white members of union households than among other white working-class voters.

The union push could have ramifications beyond Election Day. Obama had good relations with organized labor as a state senator in Illinois, and won endorsements in the middle of the primaries from, among others, SEIU, the Teamsters and the influential casino workers’ union in Las Vegas. But he has also spoken about the need for Democrats to break from union orthodoxy at times, as in his advocacy for performance-based pay for teachers. Organized labor’s priorities this year are a reassessment of trade deals and the passage of the Employee Free Choice Act, which would make it easier to organize work sites. The act would allow workers to form a union by signing authorization cards and end a company right to demand a secret-ballot election overseen by the National Labor Relations Board before a union could be certified.

Obama has in recent months softened his rhetoric on revisiting trade deals, to the consternation of some union leaders. But he continues to speak out strongly on behalf of passing the Employee Free Choice Act, and union leaders say they fully expect him to push for it if he gets elected.