First, it does not matter what the total percentage is in the country. It only matters what percentage exists in specific districts. You will see more Tea Party types in congress next year.
As long as we have statewide and national elections, what you say isn't true. Look at what happened in 2010 -- as favorable an election as the movement is EVER going to see -- when Tea Party candidates tried to run for Senate seats. They lost 3 for 4, and every one of those losses was a seat with a weak Democratic candidate that the Republicans should have won. (It's debatable whether the sole winner, Rand Paul, should be considered a Tea Party candidate.)
Moreover, if you have only 20% of the nation supporting the Tea Party's positions, then you have at most 20% of the House seats potentially winnable by Tea Party candidates, and that's only if the support is completely lopsided (i.e., almost all Tea Party supporters concentrated into 20% of the districts), which is almost certainly untrue. So the real figure is surely considerably lower.
Had they governed more to the left, they would have lost what moderate support they had in 2010 (whch wasn't much) and you would be looking at a majority in the senate.
No, that's untrue, and I can prove it. Go here for reference:
Election 2010 - Exit Poll Results - CBS News
First off, far from moderate support for the Democrats being "not much," the Dems WON the self-labeled "moderate" voters, 55-42.
Secondly, scroll down to where the exit poll asks whether people voted for Obama or McCain in the 2008 election. Note that this is evenly split, 45-45. Yet in 2008, Obama won 52% of the popular vote -- to McCain's 45%. What does this mean? It means that a lot of people who voted for Obama in 2008 DID NOT VOTE in 2010.
That completely explains the outcome. The electorate did not recoil in horror at the "socialist" overreach of the Democrats. What happened is that a lot of voters who supported Obama became disillusioned with him and stayed home two years later. Why did they do this? Not because he overreached but because he UNDER-reached, as did the Democrats in Congress.
Obama campaigned as a militant progressive in 2008, the first such candidate the country had seen since 1972, and he won by a landslide. Then he governed like a moderate Republican, and lost a lot of support. It's that simple.