LONDON — The leaders of Britain’s bold new experiment in coalition government flung off their differences and stood side by side on Wednesday, promising to act in concert to promote economic stability, reform the country’s frayed political system and demonstrate that their unlikely arrangement is more than just a hasty marriage of necessity.

The Conservative prime minister, David Cameron, and his deputy, Nick Clegg, a Liberal Democrat, pledged that their jury-rigged government, with a cabinet made up of both parties, would tackle the bloated deficit by cutting £6 billion from the budget this year.

They said that the new arrangement, which they were forced into when the Conservatives failed to achieve a parliamentary majority and looked to the Liberal Democrats to provide them with support, represented an end to the old politics of self-interest and partisan short-sightedness. And they said they would work together to tear up the old political system by establishing five-year fixed terms for Parliament — though they left an escape clause for disgruntled legislators, under which Parliament could be dissolved if 55 percent of its members voted in favor of doing so.

The moves were intended to demonstrate to a jittery, skeptical public that the new government would act swiftly and that two parties whose past relations had shifted along the spectrum from contempt to indifference could, in fact, function harmoniously and stably.