It's no secret Americans are fed up with the 'do-nothing Congress.' A new NBC News/Wall Street Journal poll shows only 14 percent of Americans believe the institution is doing a good job. So why are incumbents continuing to win their primary elections across the country?

"Perhaps the key factor is that, in reality, none of the challengers have been very strong," University of Virginia political science analyst Kyle Kondik said.

Just take a look at the results for Republican senators in this year's primaries. Sens. John Cornyn, R-Texas, Thad Cochran, R-Miss., Lindsey Graham, R-S.C., and Mitch McConnell, R-Ky., have all defeated their Tea Party-backed challengers. And most recently, Sen. Pat Roberts, R-Kan., held off his primary challenger Milton Wolf, although it was much closer than many experts predicted.

"Was Wolf a credible candidate? I'm not so sure," Kondik said. "[Sen. Lamar] Alexander is the last Republican Senator with any primary trouble."

Alexander will learn his fate Thursday when voters in Tennessee head to the polls to decide between him and state Rep. Joe Carr. It very well will be the last stand for Tea Party-backed candidates to unseat an incumbent.

The Tea Party has done a little better in House races, however. Dave Brat shocked the political world when he unseated House Majority Leader Eric Cantor, R-Va. earlier this year. The loss stunned Cantor and House Republicans and ultimately led to Cantor announcing his intent to resign from office early. In Michigan, Rep. Justin Amash, a Tea Party-backed incumbent, held off his primary challenger while securing more than 50 percent of the vote. But things didn't work out for Tea Party-aligned Rep. Kerry Bentivolio, who lost his primary to Michigan businessman David Trott.

Primary season is now winding down, and many political observers will look for trends heading into November, but if this year has taught us anything so far, it's that it's good to be an incumbent.