So kind of the SDP to keep the Conservatives in power. But,
Before the SDP was created the Conservatives were in government with a majority of 43.
After the SDP dissolved in 1988 the Conservatives won a general election in 1992 with a majority of 21.

Margaret Thatcher may have had the benefit of a larger majority because of Labour voters turning to the SDP, but that doesn't seem to have been necessary to keep power.