Mrs Gandhi will address a rally in Vijaypura, at 3 pm, two hours after PM Modi.
The BJP, which has been fighting hard to wrest the state from the Congress, tweeted:
Today, Ms. Antonio Maino is here in K'taka to save her last citadel from falling!— BJP Karnataka (@BJP4Karnataka) May 8, 2018
Madam Maino, K'taka needs no lessons from the person who was solely responsible for wasting India's 10 precious years.
And to Congress, need to remind you of your 'import' jibe? https://t.co/7NmhjuoMOM
The 71-year-old UPA chairperson has not been well for some time. She was taken ill during the rally in Varanasi ahead of the assembly elections in Uttar Pradesh and had to be admitted at a Delhi hospital.
She did not take part any campaign since, although elections were held in more than half-a-dozen states -- Gujarat, Punjab, Himachal Pradesh, Uttarakhand, Meghalaya, Tripura, and Nagaland. Last December, soon after the Gujarat elections, she handed charge of the Congress to her son Rahul Gandhi.
Sources told NDTV that Mrs Gandhi was a last minute addition to the campaign after Rahul Gandhi prevailed on her last week to address the rally. Many party leaders in Karnataka were not aware of the decision. "It is the central party's decision,'' they told NDTV.
Sources in the Congress pointed out that they wanted to cash in on the party's historical connect with Karnataka. It was in Chikmagalur, now Chikkamagalur, where Indira Gandhi won a decisive victory in the post-emergency election in 1978. Sonia Gandhi won against Sushma Swaraj from Bellary, now Ballari, in 1999.
Mrs Gandhi's joining the campaign in Karnataka is seen as yet another signal of the weight the Congress is giving to the election in the state. Karnataka is among a handful of states the party currently rules and a victory here will come as a huge morale booster ahead of next year's general election.
Up against a tough BJP campaign spearheaded by Prime Minister Narendra Modi, the Congress has deployed all its senior leaders to campaign in Karnataka, keeping Mr Siddaramaiah and the local leaders at the forefront. Yesterday, former Prime Minister Manmohan Singh was also in Karnataka as part of the campaign.
Campaigning ends in Karnataka on Thursday and the votes will be counted on May 15 - three days after Saturday's election.