Sen. Cory GardnerCory GardnerProtecting the outdoors: Three cheers for America's best idea Ex-Sen. Cory Gardner joins lobbying firm Biden administration reverses Trump changes it says 'undermined' conservation program MORE (R-Colo.) is endorsing Sen. Marco RubioMarco Antonio RubioOvernight Defense & National Security — Milley becomes lightning rod Joint Chiefs Chairman Milley becomes lightning rod on right GOP senators unveil bill designating Taliban as terrorist organization MORE (R-Fla.) for president, giving the 2016 contender his first endorsement from a sitting senator.

"Our country needs a new generation of leadership, and I believe that Marco Rubio presents this nation with the greatest possibilities and opportunities to meet the challenges of the next generation," Gardner, 41, said of Rubio, 44, on Fox News's "Happening Now."

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"After the debate in Colorado, I thought Marco did an excellent job in framing what I believe to be what Colorado and this country wants: somebody they can be excited about, somebody who is looking forward toward the future, not looking back at the past."

It's the latest boost to Rubio's campaign following a strong performance in last week's third GOP presidential debate, hosted by CNBC. A new poll on Monday also showed Rubio moving up to third place in New Hampshire.

Gardner, a rising GOP star, said Rubio is the best choice to defeat Hillary Clinton, the front-runner for the Democratic nomination.

"I believe Marco Rubio presents us with the opportunity Americans can be excited about," Gardner said, citing Rubio's positions on higher education, foreign policy and his ability to speak to a range of demographics including single mothers.

Colorado also has a growing Hispanic population, and Gardner's endorsement could boost Rubio's efforts to win over that bloc in the swing state.

Gardner told reporters at the debate last week in Boulder, Colo., that he expected some candidates to drop out of the race after the bout.

On Monday, Gardner pointed to Rubio's performance onstage with seasoned candidates in making the case that he was the GOP's best pick.

Gardner also defended Rubio's attendance record in the Senate, something that candidates including former Florida Gov. Jeb Bush have targeted in recent weeks. 

"I think if you look at others who have run for president, this seems to be a double-standard," Gardner said on Fox, mentioning then-Sen. Barack Obama's voting record while running for president. 

"The bottom line is a record of ideas," Gardner said. He said his endorsement was "not about attendance, but goals and opportunity to move this nation forward."

Gardner and Rubio have served together on several committees, including Commerce, Science and Transportation; Foreign Relations; and Small Business and Entrepreneurship. 

Gardner mentioned Rubio's tenure as Speaker of the House in Florida, his defeat of Charlie Crist in the 2010 Florida Senate race, and his debate performances.

"This isn't about looking back at past policies that have failed. This is about looking forward," Gardner said.