"Now we are in the same boat and success has to be shared."
But opponents accuse the SPS of selling out to the Kremlin, which may potentially need the Right Cause to cushion itself from anti-government feelings, especially during a global financial crisis which has hit Russia hard.
"This marks the end of these liberal organisations and of what remained of their independence in politics," Eduard Limonov, one of the leaders of the opposition group Other Russia, told Interfax.
The Right Cause is an amalgamation of the opposition Union of Right Forces (SPS) with the broadly pro-Kremlin Democratic Party and Civic Force.
The SPS - set up by pro-Western reformers - won no seats in last year's parliamentary election and did not field a candidate in the presidential election this year.
Earlier this month the SPS said it was joining the pro-Kremlin alliance because that was the only way it could survive.
Vladimir Putin, the prime minister, is hugely popular in Russia and his United Russia party dominates the Russian parliament.
Other than the Communists, opposition parties have struggled to win significant support.