Remember that Afghanistan trip Sen. Mitch McConnell took some of the GOP freshmen on last week? At the time, some conservative activists worried it was a “ploy” to co-opt the Tea Party members of the Senate. And now, interestingly, some of the same freshmen who went on the trip — Pat Toomey, Ron Johnson, and Marco Rubio — have decided not to join the Senate’s Tea Party Caucus.

In an interview with a Florida political website, Tea Party favorite Rubio said he won’t be involved in the caucus, because he thinks it will “co-opt” the whole concept of the movement:

“My concern is that politicians all of a sudden start co-opting the mantle of Tea Party. If all of a sudden being in the Tea Party is not something that is happening in Main Street, but rather something that’s happening in Washington D.C.,” he said in an interview with the Shark Tank, a Florida political website. “The Tea Party all of a sudden becomes some sort of movement run by politicians. It’s gonna lose its effectiveness and I’m concerned about that.”

What Rubio says is correct on its face. The Tea Party is a ground-up movement, and it would be completely inconsistent with its platform if Washington politicians began “running” it. But that doesn’t seem to be the point of Tea Party Caucus at all. The idea of the caucus is to take direction from the grassroots of the conservative movement and carry it out in Congress — not the other way around.

So Rubio is spinning a bit. But it’s not hard to see why. Politically, it wouldn’t be the greatest move for him to tie himself to a caucus, at least not if he wants to compromise and get things done in the Senate.

That might be why the Senate’s Tea Party Caucus hasn’t been successful in drawing members. The Hill reported that it currently has only three senators committed to attending its first meeting: Rand Paul, Mike Lee, and Jim DeMint.

The House of Representatives, in comparison, has a 30-member strong Tea Party Caucus, which was created by Rep. Michele Bachmann last year. But the Senate is also a fraction of the size of the House, meaning that senators need to compromise much more with other members in order to get legislation through.