Not only is this Rubio’s biggest endorsement to date, it’s arguably the biggest of the GOP race so far. Rubio’s pitch is that he’s a young next-gen pol who can win in swing states where Republicans have struggled recently. Now here comes Cory Gardner, all of 41 years old and fresh off the most important Senate win of the 2014 cycle in purplish Colorado, to lend his seal of approval to that message. Endorsements from members of Congress are a mixed bag in a cycle when Republican voters hate, hate, hate their leaders in Washington, but if you’re going to show one off, your safest bet is a guy who’s only been there nine months and who gave the electorate one of its biggest thrills on election night last November.

The timing is no coincidence, of course. Rubio’s expecting a bump in the polls this week (for good reason) thanks to the last debate and wants to build perceptions of momentum.

Gardner repeatedly stressed in the interview that Rubio would be the “best opportunity to beat Hillary Clinton,” the front-runner for the Democratic nomination, and said he offers a “generational choice.”

Gardner, who serves on the foreign relations committee with Rubio, also highlighted Rubio’s work on that committee and his experience as Florida’s House speaker.

Gardner also deflected questions about Rubio’s poor voting attendance record, citing a “double standard” between Rubio and past presidential contenders running from the Senate.

But Gardner took a very different stance toward voting absences last year in his election fight against Sen. Mark Udall, the Democrat Gardner unseated last November.

Yeah, go figure that politicians from Jeb Bush to Gardner are opportunistic when it comes to whining about missed votes. Big question now, though: Is Gardner the only endorsement Rubio has lined up this week or is he planning to manufacture a “young senators lining up behind Rubio” narrative to last for days? Here’s a tidbit from Mike Allen’s “Playbook” news sheet this morning:

FROM A LITTLE BIRD: Expect more conservatives to endorse Marco this week. Keep an eye on some of the younger, newer U.S. senators who like Marco’s generational appeal.

Note the plural. One obvious candidate to back Rubio is Tom Cotton, another 2014 winner who shares Rubio’s interventionist views on foreign policy. That’d be a big get. A bigger get would be Joni Ernst given her influence in Iowa. (“Look at all these people!” Ernst was heard to say when she stopped by Rubio’s booth at a multi-candidate even in Iowa this past weekend.) If Rubio has Gardner, Cotton, and Ernst in the bank, the campaign commercials write themselves: There’s a new breed of young Republicans taking over the GOP, replacing the can’t-do leadership that dominated during the Obama years, and they all think Rubio’s the man to lead them. The contrast with dinosaur Jeb Bush couldn’t be clearer, nor could the snub to Ted Cruz by his colleagues. If Rubio has Ernst quietly set to endorse, though, I’d expect that he’d hold that back for awhile until we get closer to the caucuses for maximum impact. In fact, I’m surprised they rolled out Gardner as early as they did: Rubio’s campaign so far has been quick to downplay every uptick in the polls or donor endorsement with yadda-yadda about how it’s a long race, there’s a lot of work still to be done, etc. Maybe they felt they needed to capitalize on the perception of momentum after the debate by rolling Gardner out there now. Or maybe they’re worried, given the competition from Bush and Cruz, that some of these people could change their minds on an endorsement if the campaign asks them to wait. It would be agony for Rubio if Ernst privately committed to back him and then, after a Cruz surge in the polls in Iowa, suddenly decided she couldn’t cross the voters by endorsing someone else. If you’re Rubio, maybe you think you need to lock these endorsements in by publicizing them as soon as you have their commitment.

And yes, endorsements do still matter, if only to winning the donor primary. The more it looks like influential Republicans like Gardner and Paul Singer are congregating around Rubio, the more other undecided influential Republicans will take it as proof that they won’t be throwing their money away in backing him too. That matters to a guy like Rubio who needs to show, a la Obama in 2008 versus Hillary, that he’s a real threat against an 800-pound establishment gorilla. Exit question for Rubio fans: Are there any polls out there that show him doing *significantly* better than other Republicans in winning over young voters? More importantly, is there any *series* of polls showing him trending that way? Rubio’s supposed youth appeal is taken for granted because he seems so young himself — he’s 44 but looks 30, he listens to rap, he’s a member of a growing minority demographic, etc. The most striking polls for Rubio age-wise that I know of, though, show him doing well with older Republicans, who may appreciate his old-wine-in-new-bottles appeal. I like the way Katherine Miller of BuzzFeed put it: “His appeal is to older people who think youth is important.” Any evidence that she’s wrong?