Instead of sitting out the primary elections, plenty of lawmakers are trying to help pick winners by giving to their preferred candidates "” particularly in Senate races, which tend to take shape earlier in the election cycle than House races.

Those donations can be potent because the recipients can parlay those endorsements into more financial and political support.

And the money can be pretty good, too, since many members of Congress have organized leadership political action committees, fundraising vehicles that can give $10,000 to a candidate's campaign committee "” $5,000 apiece for the primary and the general election. That's more than twice the $4,800 limit that an individual donor can give to a candidate's campaign committee.

In the race...