According to Namier and Brooke in The House of Commons 1754–1790, the right of election was in the freemen of the borough who numbered about 100. The town was known as an Admiralty borough and at least one MP was usually an Admiral.
The Earl of Sandwich was First Lord of the Admiralty from 1771 to 1782. He imposed tighter Admiralty control over the borough. This change of policy led to an independent element of the local Council supporting challengers to the Admiralty candidates between 1774 and 1780.
When party politics re-emerged in the late 18th and early 19th centuries, Portsmouth was a predominantly Whig constituency. It only once elected a Tory Member of Parliament between 1790 and 1832.
The Reform Act 1832 considerably expanded the electorate of the borough. The freemen retained their ancient right franchise, but were outnumbered by the new occupier voters amongst the 1,295 electors registered in 1832. As a result of the expanded electorate the borough became more competitive. Contested elections became the norm rather than the exception, as they had been before the Reform Act.
Candidates with naval connections continued to be frequent in Portsmouth, after the Reform Act. The borough developed into a marginal constituency, particularly in the last half century of its existence.
^Percy was re-elected to serve in the Long Parliament but was also elected for Northumberland, which he chose to represent, and did not sit again for Portsmouth
^This list follows that given by Brunton & Pennington. Cobbett lists Dowse as elected after the Civil War to replace Nicholas Weston, disabled from sitting in 1642, but Brunton & Pennington's more recent research records Weston as MP for Newtown (Isle of Wight).
^ abErle was also elected for Wareham, which he chose to represent, and did not for Portsmouth in this Parliament
^On petition, the result of the 1710 election was overturned, and Wager and Jennings were declared not to have been duly elected
^Gore was re-elected in 1747, but had also been elected for Bedford, which he chose to represent, and did not sit again for Portsmouth
^It was afterwards discovered that Legge, who had been elected in his absence, had been dead some days before his election, which was declared void
The bloc vote electoral system was used in two seat elections and first past the post for single member by-elections. Each voter had up to as many votes as there were seats to be filled. Votes had to be cast by a spoken declaration, in public, at the hustings (until the secret ballot was introduced in 1872).
Note on percentage change calculations: Where there was only one candidate of a party in successive elections, for the same number of seats, change is calculated on the party percentage vote. Where there was more than one candidate, in one or both successive elections for the same number of seats, then change is calculated on the individual percentage vote.
Note on sources: The information for the election results given below is taken from Sedgwick 1715–1754, Namier and Brooke 1754–1790, Stooks Smith 1790–1832 and from Craig thereafter. Where Stooks Smith gives additional information or differs from the other sources this is indicated in a note after the result.
The above list of members of parliament includes David Montagu Erskine as an MP in 1806, in succession to his father Thomas Erskine (who became Lord Chancellor and was elevated to the peerage as the 1st Baron Erskine in 1806). Stooks Smith does not record this election
Note (1837): Stooks Smith gives a registered electorate figure of 1,517; but Craig's figure is used to calculate turnout. Stooks Smith was the source for the number of electors voting. He classified Carter and Baring as Whigs, with Cockburn and Fitzharris as Tories.
Note (1857): Number of voters unknown. The turnout is estimated by dividing the number of votes by two. To the extent that electors did not use both their votes, the figure given will be an underestimate of actual turnout.