They should probably tell that to Hillary Clinton's campaign.
Newly released Clinton campaign emails from WikiLeaks show top aides appeared to fear Rubio's potential in much the same way the media built him up as a possible general election force to be reckoned with. It wasn't a constant theme of their emails, mind you, but it was clear how they felt about what he could become should they have faced him in the general election.
Here's Clinton pollster Joel Benenson in February: “I’m beginning to worry more about Rubio than the others. He’s playing this very smart — only one who didn’t duck like a chicken s--- on the Q of whether POTUS loves America. He has stronger right wing cred than Jeb 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.”
Adviser Mandy Grunwald responded: “He's very inspiring at his best. Feels kind of like a light weight in a lot of interviews — but I take your point.”
Now-Clinton deputy communications director Christina Reynolds said after Rubio's candidacy announcement in April 2015: “He gives a good speech, and sounded much more reasonable, populist and accessible than much of the rest of the GOP field. Felt more like an inspiring Democratic speech than a GOP candidate, outside of foreign policy, repealing Obamacare and choice. Lots of references to 'our generation' (I.e. Him and younger voters) vs. 'their generation' (them being us, Jeb, his opponents, Washington).”
One aide compared his rhetoric to President Obama's in 2008. In another exchange, a former California assemblyman who paid a political price for his own absentee voting record passes along his thoughts to Clinton campaign chairman John Podesta on that line of attack on Rubio, adding: “Many people smarter than me disagree but I would not want to run against Marco Rubio in a general election.”
Rubio has other concerns right now, of course. He's currently favored to win reelection but still is in a competitive race.
But when he decided to change course and seek another term after losing his presidential campaign, it was hard to escape the perception that he wanted to take another shot at running for president one day. After all, why would a guy who so thoroughly badmouthed the Senate and basically seemed finished with it suddenly decide to run again?
So who knows what the future holds? But it's not hard to see Rubio, if he does run for president again in 2020 or 2024, pointing to these emails as proof that Democrats are scared of him — more than basically any other Republican. His performance in the GOP primary failed to live up to the high hopes that those in the GOP establishment had for him, but even as he was coming up short, Democrats saw the potential.
That's a pretty good argument for any Republican contender to make.