The influential Koch network is throwing its support behind Colorado Republican Cory Gardner, who is one of the most vulnerable U.S. senators up for reelection next year.

Americans for Prosperity, the political arm of the vast group co-founded by libertarian billionaire Charles Koch, announced Tuesday it will help the one-term Republican defend a seat that has been deemed a toss-up.

In an op-ed, the director of AFP's Colorado operation, Jesse Mallory, said one of the reasons it's supporting Gardner is because he has not been afraid of standing up to President Donald Trump's tariffs.

"He was one of the first five GOP senators to back a proposal in 2018 to rein in the president's authority to impose tariffs unilaterally, and has called on the president to work with Congress to avoid prolonging the trade war that is driving up prices for consumers and businesses and killing jobs in industries that depend on imported components, in Colorado and across the country," Mallory said. He also praised Gardner for working with the administration on the First Step Act and tax reform. The op-ed was published in the Colorado Sun on Tuesday.

Americans for Prosperity will be hosting a rally for Gardner in Denver on June 26.

Gardner will "benefit from the full weight of AFP Action's grassroots mobilization and advertising campaigns," a senior AFP operative told CNBC, speaking on the condition of anonymity. So far, Gardner's campaign has raised just over $2 million, according to records from the Federal Election Commission. Republicans in the Senate are defending at least 20 seats next year.

For Gardner, the endorsement by the Koch-affiliated organization comes at a time when political analysts are marking Colorado's senate seat as a possible pickup opportunity for the Democrats in 2020. The Cook Political Report has put Gardner's seat under their toss-up category. Larry Sabato's Crystal Ball also considers Colorado a toss-up. At least half a dozen Democratic candidates are looking to flip Gardner's position in congress.

This won't mark the first time Gardner has received assistance from the heavily funded network.

Charles Koch himself contributed $2,600 to Gardner's first campaign in both the primary and general election. Koch's son, Chase, also donated to that run.

Gardner has also been a regular attendee at the group's donor retreats.