WASHINGTON — A White House official said Wednesday President Obama would veto Speaker of the House John Boehner's fiscal cliff "plan b," saying the middle class tax cut is an unacceptable and unbalanced solution to the fiscal cliff.

The threat represents a shift in the White House's messaging, which for weeks had revolved around the immediate passage of middle class tax cuts, while calling for additional negotiations on other elements of the fiscal cliff. Obama launched a public campaign for the tax cuts, starting a Twitter hashtag #my2k, and taking trips to Pennsylvania and Michigan to pressure lawmakers to pass the tax cuts. Now the White House is rejecting just that.

"The deficit reduction is minimal, and perversely, given its authors, solely through tax increases with no spending cuts," said White House Communications Director Dan Pfeiffer. "This approach does not meet the test of balance, and the President would veto the legislation in the unlikely event of its passage."

Boehner proposed Tuesday to cut taxes on income up to $1 million dollars — a plan previously floated by Democratic lawmakers — while leaving the rest of the fiscal cliff unchanged. Pfeiffer said the plan would give millionaires an additional $50,000 tax cut over President Barack Obama's plan, and released a fact sheet blasting the fact that it allows unemployment insurance to expire and doesn't deal with the mandatory spending cuts from the failure of the super committee. Many of those outcomes are current law that both sides have been fighting to change, but the Boehner back-up plan is limited in scope to tax rates.

The White House's plan would have left the unemployment insurance and the sequester for future negotiations, but has backed off the #my2k push to call for a comprehensive solution to the fiscal cliff deal.

"The President urges the Republican leadership to work with us to resolve remaining differences and find a reasonable solution to this situation today instead of engaging in political exercises that increase the possibility that taxes go up on every American," Pfeiffer continued.

Republican lawmakers and Obama have moved closer to an agreement in recent days, with Boehner caving on allowing tax rates to rise on the wealthiest and Obama agreeing to the "chained-CPI" which would slow the cost of living adjustment to entitlement programs like Social Security.

The veto threat is a sign that there are no alternatives to a comprehensive deal that Obama finds acceptable — essentially forcing the two sides back to the negotiating table on the "big deal" if they want to avoid the fiscal cliff.

"This is not a "plan b" because it can't pass and it has really bad policy as detailed below," Pfeiffer said in an email to BuzzFeed, referencing the fact sheet.

The White House has not proposed any alternatives to a comprehensive agreement.

UPDATE: Boehner spokesman Brendan Buck calls the Obama veto threat "bizarre and irrational," in a statement to reporters:

The White House’s opposition to a back-up plan to ensure taxes don’t rise on American families is growing more bizarre and irrational by the day. Republicans have always said a broader, ‘balanced’ plan is the ideal solution, and we have put one forward. In the absence of a ‘balanced’ solution from the President, however, we must act to stop taxes from rising across the board in 12 days. If Democrats disapprove of this bill, then there is a simple solution: amend it in the Senate and send it back to the House.