Here's what Mr. Madison had to say in Federalist 53:
A few of the members, as happens in all such assemblies, will possess superior talents; will, by frequent reelections, become members of long standing; will be thoroughly masters of the public business, and perhaps not unwilling to avail themselves of those advantages. The greater the proportion of new members, and the less the information of the bulk of the members the more apt will they be to fall into the snares that may be laid for them. This remark is no less applicable to the relation which will subsist between the House of Representatives and the Senate.
Term Limits are absolutely necessary in a Republic. As Madison notes, some legislators will become masters. In today’s world this comes from tenure and all the named and unnamed privileges afforded to long-standing legislators. This gives undue influence of citizens represented by tenured legislators (masters) over citizens represented by freshman or less tenured legislators. A Republic must protect the less represented citizen from citizens that continually elect the same legislators to gain undue power, privilege and influence in the legislative branch. The only way is to limit the term an individual can serve in the US Congress.