Logrolling is used to gather support for unpopular or uninteresting legislation through trading favors so it can be passed into law. Omnibus bills passed by the U.S. Congress are often filled with logrolled items, with legislators pledging support if pet projects are included in the bill.
Logrolling is favor trading that involves an agreement between two or more legislators who vote for each other's pet projects. First used in a legislative context by Davy Crockett, it may have come from a country custom between neighbors helping one another roll heavy logs to desired locations once timber was cut. Logrolling may be explicit quid pro quo items between legislators, or it may be implicit or tacit agreements that legislators support each other's initiatives. These agreements help legislators create large coalitions that easily pass even complex bills.
In some cases, legislators promise support in exchange for support on a future unnamed bill. Legislators with seniority may have large numbers of these owed favors, and when major controversial bills come through, these legislators may be able to wield a lot of power by delivering logrolled votes. Legislators on important committees may also be able to trade committee votes to those who are not on those committees. Logrolling practices provide some legislators with a great deal of power.