President Obama departs the White House on April 22, 2015. (Jim Lo Scalzo/European Pressphoto Agency)

Twenty-two years ago, 102 House Democrats joined 132 Republicans to ratify the North American Free Trade Agreement. Today, supporters of a new trade measure are on the hunt for votes to give President Obama more authority to negotiate multilateral trade deals. But shifting party politics and an evolving congressional map will make their task much tougher than it was two decades ago.

The Senate Finance Committee began debate Wednesday on the measure, known as Trade Promotion Authority (TPA), and while most observers expect it will pass the Senate, the vote count in the House is much tighter — and no one forecasts anywhere near 102 Democrats will support the new deal.

Most of the votes in favor of TPA will come from the Republican side of the aisle, where House Ways and Means Committee Chairman Paul Ryan (R-Wis.) is leading the campaign. But a large number of more-­conservative Republicans have voiced opposition to any deal that appears to give Obama, their political arch nemesis, any more power.

“There’s a trust issue with the executive,” said Rep. Tom Emmer (R-Minn.), who circulated a letter among House Republican freshmen in support of the trade deal. “The response that I would generally hear [among Republicans] was, ‘Hey, I don’t want to give this president, this administration, any more authority.’ ”

That means supporters will need at least some Democratic votes to pass the measure. But opposition from labor unions, which remain powerful in heavily Democratic districts, has persuaded most Democrats to publicly oppose fast-track authority. In the last Congress, 127 Democrats signed a letter opposing TPA in a campaign spearheaded by Rep. Rosa L. DeLauro (D-Conn.). This year, nine newly elected members signed a similar letter circulated by Rep. Ruben Gallego (D-Ariz.).

That leaves only 51 Democrats who have not publicly stated their positions. And few of them are willing to publicly offer their support. The White House is largely whipping up support for its own bill, along with Sen. Ron Wyden (Ore.), the bill’s lead Democratic sponsor in the Senate.

In a statement Wednesday House Minority Leader Nancy Pelosi (D-Calif.) said, “House Democrats have been working to find a path to yes for the Trans-
Pacific Partnership.” She acknowledged that the top Democrat on the Ways and Means Committee, Rep. Sander Levin (D-Mich.), was opposed to the fast-track proposal and would try to alter it with an amendment on Thursday. Pelosi said the White House has provided many briefings in recent weeks.

“What we will do and what we have been doing is working closely — primarily through Democrats but also in conversations with Republicans — to encourage some bipartisan agreement around TPA legislation,” White House press secretary Josh Earnest said Monday. Cabinet officials, including Labor Secretary Thomas Perez, Commerce Secretary Penny Pritzker, Agriculture Secretary Tom Vilsack and Treasury Secretary Jack Lew, have begun making calls to Democrats.

Observers say the path to a trade deal is significantly more difficult than it was in 1993, when NAFTA passed on a vote of 234 to 200. That’s because the Democratic caucus looks significantly different today than then: Of the Democratic votes for NAFTA, 72 came from members who represented Southern and border states.

About half those seats, almost entirely in the South, are now held by conservative Republicans, many of whom who are, like those Emmer encountered, reluctant to give Obama any additional authority. Without those votes, NAFTA would have failed.

“The Republican gerrymander has eliminated a bunch of pro-fast-track Democrats,” said former congressman Martin Frost, a Texas Democrat who voted for NAFTA, then lost his House seat after a mid-decade redistricting. As for the remaining Democrats, he added: “Labor is still a significant force in a number of Democratic districts. I think very few Democrats will cross labor on this issue.”

The Democratic president who spearheaded the NAFTA campaign, Bill Clinton, had advantages that Obama does not. Clinton’s whip team, led at the time by Rahm Emanuel, then the White House political director, could trade earmarks to secure votes. Obama has no such leverage.

Even then, recalls William A. Galston, a former top domestic policy adviser to Clinton and now a senior fellow at the Brookings Institution, the battle to pass NAFTA was a “three-month, all-hands-on-deck struggle.”

But there is a path: Among the 51 Democrats who have not publicly rejected TPA, 37 come from districts where manufacturing jobs represent a smaller percentage than the national average, making them potentially less susceptible to pressure from labor. In an average congressional district, manufacturing jobs make up 10.6 percent of all positions; 22 Democrats represent districts where manufacturing accounts for fewer than 8 percent of jobs.

Where Democratic votes for trade once came from the South and border states, Frost said TPA’s success or failure will rest with members from the West Coast, whose states benefit the most from trade with Pacific nations. Of those members who have not said they oppose TPA, 17 are from Washington, Oregon and California.

Galston said the new realities of campaign politics could change the Democratic calculus. As Democrats find more support in highly educated and racially diverse populations, trade deals may no longer be the third rail they once were.

“This new Democratic Party, made up of a lot of people who are interested in social issues, is not as foursquare against trade agreements,” he said.

Conversely, Republicans are performing better among white working-class voters, giving them control of many more districts where manufacturing makes up a disproportionately large number of jobs. One hundred thirty-seven Republicans represent districts with higher-than-average numbers of manufacturing positions; in 57 of those districts, manufacturing makes up more than 15 percent of all jobs.

The white working-class voters who hold those jobs are much more likely to feel the economic recovery has left them behind, fueling some of the opposition to a trade deal.

In a Washington Post-ABC News poll conducted in January, 39 percent of Americans said the country was headed in the right direction; among whites without college degrees, that number was just 22 percent.

“When you hear ‘fast-track’ coming from Washington, the first thing you think is, ‘These guys are pulling a fast one,’ ” said Emmer, the Minnesota Republican who backs TPA.

Despite the shifting political alliances that complicate the debate over trade, the parallels between NAFTA 22 years ago and TPA today show that angst over trade deals remains deep among voters — and politicians — in both parties.

“The sense of vulnerability that parts of the economy, and therefore parts of the parties, are experiencing are more of a constant than a variable,” Galston said.