Next year was supposed to be a prime opportunity for Republicans to retake the Senate. And for a while, everything seemed to be breaking their way: a wave of Democratic retirements, a fluke in the electoral map that put a large number of races in states that President Obama lost, a strong farm team of conservative Senate hopefuls from the House. Related Then the government shut down. Now, instead of sharpening their attacks on Democrats, Republicans on Capitol Hill are being forced to explain why they are not to blame and why Americans should trust them to govern both houses of Congress when the one they do run is in such disarray. Complicating the prospects, the grass-roots political force that has provided so much of the energy for conservative victories over the last four years — the Tea Party — is aggressively working against Republicans it considers not conservative enough.