Addressing a meeting of several opposition parties on the presidential poll's eve, she said the election represents "a clash of ideas" and "a conflict of disparate values".
Meira Kumar and Gopalkrishna Gandhi, who are presidential and vice-presidential candidates of major opposition parties, were present. Vice-presidential election will take place on August 5. Both the elections take place through secret ballot and electors are not bound by any whip.
Janata Dal-United, which is supporting ruling NDA's candidate Ram Nath Kovind in the presidential election but Gandhi for vice president, did not attend the meeting.
Alluding to the ruling BJP-led National Democratic Alliance having an edge in both elections, Sonia Gandhi said: "...the numbers may be against us. But the battle must be fought and fought hard. We cannot and must not let India be hostage to those who wish to impose upon it a narrow-minded, divisive and communal vision."
"This election represents a clash of ideas, a conflict of disparate values. The election demands a vote of conscience to preserve the India that the Mahatma and that illustrious generation of freedom fighters, joined by thousands upon thousands of ordinary men and women, fought for," she added.
Noting that both the President and Vice-President were entrusted to discharge the duties in a manner that preserves, protects and defends both the Constitution and the law, "both of these are, sadly, under siege today", she said: "We must ensure, therefore, that the best minds and the best servants of India stand at the helm."
"Meira Kumarji and Gopalkrishna Gandhiji will be the best possible President and Vice-President to steer our society through the crisis that has beset our country today," she said.
"The presence of so many of you from so many different parties to support our distinguished candidates confirms that the fight for an inclusive, tolerant and pluralistic India is being truly waged. It is a fight we will never give up," she said.
Sonia Gandhi said both Meira Kumar and Gopalkrishna Gandhi have "through their work and their lives they have also advocated those very values and principles that have defined and guided India for the last 70 years".
She singled out Meira Kumar's role as Lok Sabha Speaker as a "duty she performed with great forbearance and with immense dignity", while saying Gopalkrishna Gandhi has distinguished himself as a public intellectual, civil servant, diplomat, author and Governor, and who "has not failed to stand up to those who promote fear as they seek to suppress freedom and diversity".