Lord Kilclooney has been accused of racism for describing vice president-elect Kamala Harris solely as "the Indian".
There were calls for action against the former Ulster Unionist Party deputy leader over his tweet about Ms Harris, who is the first black and Asian-American person to be elected to the role.
The crossbench peer, who previously deleted a tweet describing then-Irish taoiseach Leo Varadkar as a "typical Indian" after condemnation, insisted there was "nothing racist" in his latest message.
Ms Harris, a senator from California, made history by being the first woman to be elected vice president when voters chose her and Joe Biden to replace Donald Trump in the White House.
On Monday, Lord Kilclooney tweeted: "What happens if Biden moves on and the Indian becomes President. Who then becomes Vice President?"
Conservative MP Simon Hoare, who is chair of the Commons Northern Ireland affairs select committee, wrote that the message is "Bad. Rude. Racist. Appalling" and said he had submitted a formal complaint to the Lord Speaker's office.
Labour's leader in the House of Lords, Baroness Smith, added: "I am so angry about this comment. It is despicable and beneath contempt, and totally unacceptable from anyone - but especially from someone in Parliament.
"I will be making an official complaint to both the commissioner for standards and the Lords Speaker."
Labour shadow minister Wes Streeting said "action must be taken" over the peer's tweet.
"He did it before to Leo Varadkar and now he's done it to Kamala Harris. This sort of racism would be unacceptable from anyone, but from a member of the House of Lords it beggars belief," Mr Streeting tweeted.
Labour's shadow justice secretary David Lammy added: "She is called Kamala Harris. Please send this awful comment straight back to the dark ages."
Comedian Nish Kumar tweeted: "This guy is a life peer. What a country."
Lord Kilclooney denied that his message was racist.
He told the PA news agency: "The first thing is to get it in perspective, the criticism is minor, the support is massive - I've never had so many Twitter followers in one day, ever.
"I'm very fond of India myself, I'm a member of the British India all-party group, I have two Indians (tenants) in my flats here in London and there's nothing racist in it whatsoever.
"She's proud of her Indian roots just as Biden's proud to say he's Irish."
He justified his previous tweet about Mr Varadkar by saying "I didn't know his name at the time" but, asked if he will delete his latest message now he knows Ms Harris's name, he said: "No, not indeed. Because she's proud of her Indian background."
The 82-year-old peer, who is no longer a member of the UUP, said that he did not try to find out her name online before writing the tweet.
Asked why he did not use a search engine before sending the message, he replied: "That's not your concern."
Ms Harris, the daughter of Jamaican and Indian immigrants, will become the highest-ranking woman to ever hold an elected office in the US when the Democrat is inaugurated on January 20.
In 2018, Lord Kilclooney tweeted "typical Indian" in response to a message about criticism of Mr Varadkar, whose father is from India, during a visit to Northern Ireland.
The House of Lords is yet to comment.
