The latest batch of emails related to Hillary Clinton and her campaign revealed the Democratic nominee was most worried about Sen. Marco Rubio becoming the GOP presidential nominee.

Throughout 2015, emails show that Clinton's campaign staff considered Rubio the biggest threat, and sought ways to ensure he wouldn't become the nominee, even before with his announcement speech in April of that year.

In February, Clinton pollster Joel Benenson warned Team Clinton about the potential of a Rubio run.

"I'm beginning to worry more about Rubio than the others," Benenson wrote. "He's playing this very smart – only one who didn't duck like a chicken s*** on the [question] of whether POTUS loves America. He has stronger right wing cred than Jeb [Bush] and he's finding a way to the middle enough for now and he will be the most exciting choice to Republicans. Could pose a real threat with Latinos etc."

After Rubio announced his presidential campaign, Team Clinton got nervous.

"He gives a good speech, and sounded much more reasonable, populist and accessible than much of the rest of the GOP field," wrote Clinton's deputy communications director, Christina Reynolds. "Felt more like an inspiring Democratic speech than a GOP candidate, outside of foreign policy, repealing Obamacare and choice."

Reynolds then flagged several soundbites and notes from Rubio's announcement speech, including his mention of "our generation" as opposed to the generation of Clinton, Jeb Bush and other career politicians.

Clinton staffer Tyson Brody also sent an email comparing Rubio to President Barack Obama in 2008.

"Just did this really quickly, but it's interesting to compare/contrast with Obama 08," Brody wrote, before including quotes from Rubio's speech and an old Obama speech from 2007 that were similar in tone and optimism.

By May, Team Clinton discussed whether to attack immigration comments made by Rep. Steve King, R-Iowa, fearing that Rubio would make the same attack and then they wouldn't be able to draw a distinction with him on immigration.

"It always makes sense to whack Steve King. And this is particularly outrageous and bigoted and (fill in the bad word)," wrote senior campaign adviser Jake Sullivan. "But the question from you and the comms team is whether hitting King takes away from our effort to get a clean contrast with Bush/Rubio et al. They would also attack King, so it sort of lets them off the hook."

Sullivan had been responding to consultant Mandy Grunwald, who asked about drawing "a great contrast" between Clinton and King.

Then in July, the Clinton campaign spent hours crafting a response to a tweet from Rubio criticizing the Obama administration's actions in Cuba. The first email about the tweet was sent at 10:31 a.m. by Jeremy Massey, and by 5:45 p.m., after numerous tweet response suggestions from various staffers, the campaign was still trying to get a response approved.

In October 2015, the campaign began looking for ways to use Rubio's lack of votes in the senate against him, even though the primaries weren't over. In one email, Clinton campaign chair John Podesta forwarded an email he had received from Nathan Fletcher, a former GOP assemblyman from California who had lost a mayoral race (he left the GOP and ran as unaffiliated) in part due to his poor voting record.

In the email, Fletcher says specifically that he "would not want to run against Marco Rubio in a general election." He also noted that Rubio's missed votes had already been raised but that things like that didn't really resonate with voters. But, Fletcher said, his opponents phrased the issue as him "not showing up to work" while collecting a government salary.

"They very creatively (although not honestly) counted the days I missed in Sacramento even when there were no votes taking place to say I wasn't there XX% of the days," Fletcher wrote. "We initially ignored the hits because I had always been rated the 'hardest working' and 'most effective' legislator, etc. By the time we realized the damage done it was too late to respond."

While Team Clinton clearly feared Rubio, they privately hoped the GOP would nominate Donald Trump, which it did. On March 13, 2016 – just two days before Rubio dropped out of the race – former Hill staffer Brent Budowsky sent an email to Podesta saying he was "petrified that Hillary is almost totally dependent on Republicans nominating Trump."

As early as April 2015 – two months before Trump announced he was running for president – Team Clinton was hoping he would run. A memo sent to campaign staff said they "need to be elevating the Pied Piper candidates so that they are leaders of the pack and tell the press to [sic] them seriously."

Opposing candidates always fear one potential opponent more than others. In 2011, ABC reported top Democrats as saying John Huntsman would be the most dangerous challenger to Obama. Of course, Republicans nominated Mitt Romney that election, and lost.

Ashe Schow is a commentary writer for the Washington Examiner.