WASHINGTON — Vice President Mike Pence on Tuesday declared the United States’ “unwavering support” for planned mass protests in Venezuela against President Nicolás Maduro, the most explicit backing yet by the Trump administration as opposition leaders try to unseat Mr. Maduro.

The declaration, the first in which Mr. Pence used a video to directly urge Venezuelans to take action, came after months of statements by senior Trump administration officials, including several strongly worded ones from the vice president, that said Mr. Maduro’s rule was illegitimate.

“We are with you,” Mr. Pence wrote on Twitter on Tuesday in both Spanish and English, adding, “We stand with you, and we will stay with you until Democracy is restored and you reclaim your birthright of Libertad.”

The post, which came as Venezuelan opposition leaders were calling for citizens to take to the streets on Wednesday, was accompanied by a video in which Mr. Pence said he and President Trump were united with them in their “call for freedom.”