The GOP establishment won the first round of a series of primaries this month that could set the party up to retake control of the U.S. Senate in the midterm elections.

The victory by Thom Tillis (R) in North Carolina's Republican Senate primary — and several closely watched lower profile wins for the establishment in House races — allowed Republican insiders to breath a sigh of relief that they might not put up unelectable candidates in the fall like they did in 2010 and 2012.

Rob Christensen of the News & Observer noted that it "was a good election for the big boys in Washington, for deep pockets and for party connections. Voters tended not to be in a rebellious mood Tuesday, generally giving their votes to candidates who were well within the mainstream of their party. The Republican candidates who won tended to be deeply conservative, well-financed, but also politically seasoned and well-connected to their national parties. This was an insiders' election."

But there's another lesson in these results that should give Republicans pause as they look towards the fall elections: It's very hard to beat an incumbent.

As Morning Line notes, incumbents went 36-for-36 last night: "Just two incumbents got real scares — GOP Rep. Walter Jones in North Carolina, who survived yet another challenge with a six-point victory; and Ohio GOP Rep. Dave Joyce, who held off a tea party opponent by 10 points."

Yesterday's primaries showed that big money still rules and no one can raise money in bigger sums than incumbents.

The biggest thing Democrats may have going for them is that they currently hold the seats that Republicans are trying to win.