The government of Colombia needs to make extensive changes to its laws and bolster its protection of union members before a free trade agreement moves forward, a key Democratic lawmaker said Tuesday in the most explicit statement yet of the hurdles facing the proposed Colombia-U.S. trade deal.

The comments by Rep. Sander M. Levin(D-Mich.), ranking member of the Ways and Means Committee and an important voice on trade issues, shows the quandary faced by the Obama administration. It is trying to push a recently negotiated agreement with Korea through Congress amid Republican demands that long-pending deals with Colombia and Panama move as well.

Levin’s comments in a speech at the Peterson Institute for International Economics raise the prospect of Democratic opposition to the Colombia deal in particular. He urged the administration to pursue separate action on the different agreements, and to pressure the Republican House leadership to vote on Korea now. The work to be done on the other agreements, particularly Colombia, can’t be rushed, Levin said.

“They need to change their laws and they have to take steps” to ensure that workers can organize and that violence against union leaders is prosecuted, Levin said. Although Levin said he believes the new administration of Colombian President Juan Manuel Santos is sincere about improving conditions for unions in the country, the reforms that are needed are complex and have been stymied in the country’s legislature before.

They include, for example, reform of Colombia’s labor laws to restrict the use of what Levin called “shell companies” that insulate larger corporations from collective bargaining agreements, and prosecution of several high-profile cases of violence against union leaders.

Until those and other actions are taken, Levin said the administration should not submit the Colombia deal for ratification.

“The president has been clear that it is his goal to send three trade agreements to Congress,” said Carol Guthrie, a spokeswoman for the U.S. trade representative’s office, adding that the timing would be determined by the pace of negotiations with each government.

Levin’s talk reflects the still-deep national divisions over trade, with traditional Democratic voices arguing that existing agreements have cost the U.S. manufacturing jobs, and organized business groups and Republican leaders calling for faster action on a variety of proposed trade treaties.

U.S. trade officials have held several rounds of talks with the Santos administration in recent weeks. Although they have said they hope to complete negotiations by the end of the year, they have not detailed what they are asking Colombia to do to satisfy what U.S. Trade Representative Ron Kirk has referred to as “core American values” on labor and union rights.

Like the proposed Korea deal, the Panama and Colombia free-trade pacts were negotiated by the Bush administration, but have languished without congressional ratification.

Negotiations between the U.S. and Korea focused on commercial issues, and Kirk was able over several months of talks to win significant concessions, including tariff and regulatory changes that prompted the United Auto Workers to offer an important union endorsement of the pact. Levin also supports it.

The issues surrounding Colombia, however, are a more direct test of Obama’s campaign pledge to retool U.S. trade policy so that it does not extend free-trade privileges to countries with lower labor, environmental and other standards. While the approach has a moral dimension, it is also meant to keep U.S. workers from competing against countries where goods can be produced more cheaply because the laws are lax.