The two agencies have a shared history, as the names suggest, but are now bitter rivals. Originally there was one agency, founded in 1970 by two Iraqi Jewish emigrants to London, Maurice and Charles Saatchi. The agency became a household name when it was hired by former prime minister Margaret Thatcher to run the Conservatives' successful 1979 election effort, a billboard campaign under the slogan "Labour Isn't Working" featuring an unemployment line.
Maurice and Charles were ousted from their own agency in 1995 in a boardroom coup. Furious, they formed a new, rival agency, M&C Saatchi. However, the power of the original Saatchi & Saatchi brand has always outshone the brothers themselves, and M&C Saatchi has toiled in the original agency's shadow ever since.
M&C Saatchi retained the Conservative Party as a client, however, and produced the infamous "Demon Eyes" election campaign for them which featured an image of then-prime minister Tony Blair looking devilish. Labour, and Blair, won that election.
Watching with interest from the sidelines will be WPP (WPPGY) CEO Martin Sorrell. Until 1984, Sorrell was Saatchi's CFO. He left the company to found WPP, which is now the largest ad agency holding company in the world.
It's not clear whether Maurice and Charles will have a direct role in the Conservative campaign as they stepped back from agency management in the mid 2000s after more than three decades in the business.
The other man to watch in the battle is current Saatchi CEO Kevin Roberts. He is obsessed with the Saatchi brand, and often refers to his shop as "the most famous ad agency in the world." He will be personally furious if the smaller, upstart, less well-known Saatchi brand can steer an enemy client to victory.
Initially, the Conservatives had hired Euro RSCG for their campaign, but a billboard they produced for the party featuring an eerily airbrushed Tory leader David Cameron has become a national joke. The race, expected to end in May, is now much closer.