The government of Colombia has agreed to better protect union members and vigorously prosecute those responsible for violence against them, potentially paving the way for the Obama administration to seek congressional approval of a free trade treaty with the South American country.

The labor protections — included in an agreement to be signed Thursday in a meeting between President Obama and Colombian President Juan Manuel Santos — commit Colombia to a strict timetable of steps to be taken by the end of the year.

The government agreed to eliminate by July 30 a backlog of requests by union members for special protection; to hire 95 police investigators by the end of the year to help prosecute anti-union violence; and to implement by June, two years ahead of schedule, regulations that fine businesses for using subsidiaries and other affiliates to thwart collective bargaining.

The decision may shore up Obama’s standing in a region where he has pledged greater U.S. engagement but has been short on follow-through, according to analysts familiar with U.S.-South American relations.

But it is likely to touch off intense politicking in Washington. Major unions and Democratic leaders criticized the administration, arguing that until Colombia shows it can reduce the steady violence against union leaders it should not be granted free trade privileges. The U.S. Chamber of Commerce and other major business groups lauded the announcement, and leading Republicans urged fast action.

Senior administration officials did not commit to a time frame for submitting the Colombia agreement to Congress, and they said the agreed-upon labor rights measures would have to be in place before the free trade pact takes effect. The concessions offered by Colombia, administration officials said, would create a “sound basis” for talks with Congress about the Colombia agreement and several other pending trade issues, including a free trade pact with Korea and payments to U.S. workers whose jobs have been disrupted by imports.

Obama has been under pressure from Republicans to show tangible economic support for the closest U.S. ally in Latin America. During his first trip to Latin America last month, Obama spoke about building trade links with an increasingly affluent region that already receives three times as much in U.S. exports as China.

“He justified the trip to Brazil, particularly, in terms of creating export jobs,” said Michael Shifter, the president of the Inter-American Dialogue, a Washington policy group. “And it just seemed like a contradiction that the president was talking about his commitment to the trade agenda while not moving forward with these trade deals. That was seen as incongruent and hard to explain.”

As trade deals with Colombia and Panama languished during the George W. Bush administration and the first years of Obama’s, China’s economic ties to the region grew stronger. China is now the biggest trading partner of Brazil and Chile.

Colombia remains the world’s most dangerous country for trade unions, with 51 union activists slain in 2010. In the 1990s, 200 or more were slain in single years, but the number dropped to 39 in 2007 before rising again, according to the National Union School, a labor research group in Medellin whose numbers are cited by the United Nations and the State Department.

“There are sectors of the economy where unionism was exterminated and others where it has been reduced to its minimal expression, which doesn’t allow workers to participate in discussion of labor issues,” said Luciano Sanin, director of the National Union School, a Colombian think tank. He said that just 4 percent of Colombia’s workforce is unionized.

Sanin said the attorney general’s office has taken important steps by creating special investigative teams to clear hundreds of unsolved cases. But with nearly 3,000 union activists slain in Colombia in the past quarter-century, and prosecutors short of resources, the vast majority of cases remain unsolved, Sanin said.

He said there were 74 convictions from 2002 to 2010, with an additional 122 defendants pleading guilty. “With this pace for the investigations and convictions, it’s going to take another 40 years,” said Sanin.

Juan Carlos Forero, the deputy attorney general, said 19 prosecutors in a special unit responsible for clearing unsolved killings of union activists has a workload of 1,387 cases involving 1,885 victims.

Some Democrats who have been critical of Colombia’s efforts to resolve the slayings said they do not believe the proposals by Santos and Obama go far enough.

“The bottom line is good intentions are not change,” said Rep. Jim McGovern (D-Mass.), who has traveled often to Colombia. “And what we’re interested in is real change.”