The power of Congress has corrupted it, and must be reduced
A line-item veto is a transfer of power from Congress to the President. This would be the first such transfer in our history. The only other Constitutional changes that are comparable are the direct election of Senators and Presidential term limits.
The 17th Amendment stripped state legislatures of their power to elect Senators. They deserved it. A seat in United States Senate was a prize that for sale to the highest bidder. The corruption of the state legislatures caused their loss of power.
President Franklin Roosevelt showed that a skilled politician could become President for life. The 22nd Amendment was a repudiation of the concentration of power in the President. Roosevelt’s refusal to give up power meant his successors were term limited..
And so it is with Congress and the line item veto. They have richly earned their comeuppance, giving us 21 trillion reasons to reduce their spending powers.
A political campaign for the line-item veto is a campaign against Congress, and, effectively, its leaders. As such, right now it’s hard to see how it could lose. Whoever leads such a campaign will be richly rewarded, politically.