Groups on both the Left and Right lobbied furiously Wednesday as the Senate Finance Committee took up Trade Promotion Authority legislation.

Liberal lawmakers sought to attach various provisions to the legislation, also known as "Fast Track," to mitigate the effects of free trade on certain industries. Conservative groups warned that including those amendments could cause them to oppose the legislation.

Sen. Sherrod Brown, D-Ohio, is negotiating with Sen. Ron Wyden, D-Ore., a co-sponsor of the underlying bill, to include Trade Adjustment Assistance, a federal program that helps workers who lose their jobs because of increased imports.

During a scheduled markup session Wednesday, Brown said he had other amendments as well. "I look forward to debating them today, and into the evening," he said before the committee recessed for the morning.

The committee was expected to reconvene Wednesday evening. The initial delay was caused by Sen. Bernie Sanders, I-Vt., a fierce critic of Fast Track, denying the Senate unanimous consent to move forward with it.

Michigan House Democrats Dan Kildee and Dennie Dingell said in a press conference Wednesday that the amendments, particularly one that would impose sanctions on countries that deliberately weaken their currencies, are crucial to maintaining Democratic support.

At the same time, conservative organizations have warned that amending Fast Track at all could cost their support. The Club for Growth announced Wednesday that it would oppose Fast Track if Democrats succeed in watering down the legislation. The group warned that it might even make opposing the legislation a "key vote" if it arrives on the Senate floor with the Democratic amendments.

"The.. currency amendment has been rightly called a protectionist 'poison pill' by the Wall Street Journal. It would incite a trade war that is antithetical to economic freedom. In addition, we oppose Sen. Sherrod Brown's three amendments to plus up Trade Adjustment Assistance (TAA), which would only serve to feed more tax dollars into yet another government welfare program," said Club for Growth President David McIntosh.

Republican leaders are trying to tamp down concerns over the legislation. House Speaker John Boehner's office released a "Myths v. Facts" press release on Fast Track, saying "opponents are relying on a campaign of fear and misinformation to support their case against this bipartisan initiative."