There were many factors that led UPA to retain its power in General Elections 2009. The left parties led by Karanth have strongly opposed the Nuclear deal with US and withdrew support to the UPA prior to the elections. Samajwadi party and BSP, for the reasons best known to them, have bailed out the Govt. in the No confidence motion and the hype was given that the Nuclear deal would help the country a lot. Added to this the then Speaker of the Lok sabha who was a strong CPM leader refused to quit the post and supported the deal. Thus Manmohan Singh became very strong in implementation of the deal and got endorsed as a successful and determined world leader.
The above factor had the influence in the General election and weakened the Left parties in particular. Added to this, the BJP could not project a strong leader other than Advani and Mamohan's projection as a dynamic leader overshadowed everyone. Congress became more stronger in the erstwhile AP and bagged 35 seats out of 42 because of it dynamic leader Late YSR which helped them to have the control among the UPA partners. Had the party failed in AP at that time, the situation in the National scenario would have been different. The UPA-II became more weak because of various scams and the split in Congress in AP and also the unilateral decision to bifurcate the state of AP without proper working of modalities. The country found a new leader emerged in the form of Narendra Modi which was the root cause for its debacle and all these factors led them to meet their Waterloo in 2009 elections.