ORANGE, Va. — In June, after he had written a scorching opinion article seeking to constrain the president’s unilateral power to make war, Senator Tim Kaine of Virginia, one of Barack Obama’s earliest supporters, buttonholed the commander in chief at the White House for what he called “a spirited discussion.”

The militants of the Islamic State were pouring across the Syrian border into Iraq, and seizing cities where so much American blood and treasure had been spilled. But Mr. Kaine said he told the president in no uncertain terms that if he intended to go to war, he would have to ask Congress’s permission. President Obama politely but firmly disagreed.

They have been battling ever since.

Mr. Kaine is an unlikely leader in the fight between Congress and the White House over a declaration of war. Genial and junior, the former Virginia governor was on Mr. Obama’s short list for the vice presidency in 2008. He became Mr. Obama’s handpicked Democratic Party chairman, then his handpicked senatorial candidate after Senator Jim Webb, a Democrat, announced his retirement in 2011.

But Mr. Kaine established his position in May, when he introduced legislation to repeal the 2002 authorization of force that paved the way for the invasion of Iraq. Then in September, he drafted his alternative, a narrowly tailored resolution to give Congress’s blessing to a war against the Islamic State, with a one-year time limit and explicit language ensuring the mission could not expand, either to ground troops or to other targets.