Is this British politics, ie. Gladstone, Palmerstone etc?
What I've been taught to do is to start with an introduction that begins with ''This question focuses on...'' followed by a paraphrase of the question, such as ''the reasons for liberal dominance in the period 1846-1865''. Then define key terms, such as dominance (eg, ''the popularity of the Liberal Party during the period, which enabled them to maintain political power''). You should probably explain the significance of the dates, although you could incorporate this in your paraphrase. Something like ''...maintain political power from the time the Corn Laws were repealed to the end of Palmerston's ministry''. Finally, say what you are going to look at in the essay, and end with a transition sentence: ''In order to address the question, several factors which may have contributed to liberal dominance must be examined...'' Then say which one you are going to address first, and end with something like ''which will now be examined''. Then move onto your first paragraph, which will be all about the thing you just introduced.
As you said, it is vital to do a mini-conclusion at the end of each paragraph, and use a suitable transition sentence to move onto the next one. In your conclusion, tie all of these mini-conclusions together and provide your final answer. The best way to do that in this instance is to provide a ''batting order'', ie. saying which was the main reason for liberal dominace, which was the next most important, etc.
Hope this helps, although I imagine that in the time it has taken me to type this out someone has already provided a far more concise and legible answer...