In Vox, Lee Drutman explains why Congressional term limits are a bad idea.
Since 15 states do have term limits, we actually can know something about their effects. And the political science literature here is pretty unequivocal. Term limits are the surest way to weaken the legislative branch and empower the executive branch. Term limits are also a great way to empower special interests and lobbyists. Basically, what term limits do is shift power toward those who are there for the long haul.
This result has been replicated multiple times. In one study, a post-term-limits respondent said that after term limits, “agencies [do] what they want to. [One bureaucrat told me] we were here when you got here, and we’ll be here when you’re gone.” As the authors of this study note, “Legislative oversight is the venue of specialists. A term-limited legislature tends to be populated by generalists, who lack the accumulated knowledge to exercise oversight effectively, if they even recognize it as their responsibility.”
Term limits also strengthen the power of lobbyists and interest groups for the same reason. In term-limited states, lawmakers and their staff have less time to build up expertise, since they are there for a limited time. But like the executive agencies of the state government, lobbyists and interest groups are also there year after year. They are the true repeat players building long-term relationships and the true keepers of the institutional knowledge. This gives them power.
I forget where but I heard recently that 50% of the House is under 6 years and 70% of the Senate is under 12 years, the numbers that Trump recently suggested.