Their donors. In the USA members of congress spend an embarrassingly large amount of their time on the phone to their donors ensuring they are happy enough to fund their next run.

January 2013: https://www.huffpost.com/entry/call-time-congressional-fundr...

> A PowerPoint presentation to incoming freshmen by the Democratic Congressional Campaign Committee, obtained by The Huffington Post, lays out the dreary existence awaiting these new back-benchers. The daily schedule prescribed by the Democratic leadership contemplates a nine or 10-hour day while in Washington. Of that, four hours are to be spent in "call time" and another hour is blocked off for "strategic outreach," which includes fundraisers and press work.

April 2016: https://www.cbsnews.com/news/60-minutes-are-members-of-congr...

> Rep. Rick Nolan: Well, both parties have told newly elected members of the Congress that they should spend 30 hours a week in the Republican and Democratic call centers across the street from the Congress, dialing for dollars.