WASHINGTON — The Obama administration is rushing to reach a new deal intended to lower barriers to trade with a dozen Pacific Rim nations, including Japan and Canada, before the end of the year.

But the White House is now facing new hurdles closer to home, with nearly half of the members of the House signing letters or otherwise signaling their opposition to granting so-called fast-track authority that would make any agreement immune to a Senate filibuster and not subject to amendment. No major trade pact has been approved by Congress in recent decades without such authority.

Two new House letters with about 170 signatories in total — the latest and strongest iteration of long-simmering opposition to fast-track authority and to the trade deal more broadly — have been disclosed just a week before international negotiators are to meet in Salt Lake City for another round of talks.

“Some of us have opposed past trade deals and some have supported them, but when it comes to fast track, members of Congress from across the political spectrum are united,” said Representative Walter B. Jones Jr. of North Carolina, who circulated the Republican letter.