He said: "With significant Tory and UKIP power blocks in the next parliament, we will be the left-of-centre backbone of a Labour administration.
"Along with the SNP and Plaid Cymru, (we) will ensure that the next Labour government remains true to its values."
Dr McDonnell said that, while his party had always worked closely with Labour, the SDLP was not tied to any other party.
He said its MPs would be elected on their own manifesto.
"During the last Labour government, we were unwavering in opposition to the Iraq War," he said.
"Our MPs voted against the cut in the 10p tax rate. We refused to accept 42 day detention.
"These were all issues that Labour supported."
The SDLP leader added that his party would "categorically refuse" to support David Cameron and the Conservative Party.
SDLP leader Alasdair McDonnell has said if Labour leader Ed Miliband becomes the next prime minister "it will be with our support, on our terms, that he takes power".