Politically ambitious but Jewish, which prohibited him from standing for Parliament (as was the case for Catholics and non-Conformists), Lopes converted to Christianity in 1802. 

Having purchased Maristow House in Devon (1798) as his family seat he became Member of Parliament for his home borough, before representing other constituencies. He spent part of his wealth in purchasing influence in many Parliamentary boroughs. 

The most notorious of these is Grampound one of the most notorious of the rotten boroughs.  In 1819, Lopes was discovered to have bribed the voters in two separate constituencies in the 1818 general election. Such corruption was far from uncommon but there was increasing pressure for electoral reform and but reformers were looking for a cause celebre to give prominence to their campaign. 

It is possible that, as a foreign-born Jew, Lopes became an ideal villain to give support for their cause.  It transpired that Lopes had spent £3000 bribing Barnstable voters and his election was declared void but even more notoriously it was found that he had behaved similarly in Grampound, which returned two members in Cornwall and he was fined £1000 and jailed for two years. 

Grampound was permanently disenfranchised and its Commons representation, an extraordinary two seats, was transferred to Leeds.