Tonight, President Obama, who rose to the presidency on the strength of stirring speeches, goes back to the well with a prime-time address urging Congress to authorize an attack on Syria.

He admits it’ll be “a heavy lift.” And how: per the Washington Post’s latest whip count in the House, even if all 170 undecideds break their way, the administration won’t be within shouting distance of a majority.

That’s good, because the Authorization for the Use of Military Force that’s on the table deserves to fail. It’s TARP with Tomahawks.

The provisions purporting to restrict the president to a brief, “limited and tailored” war are too weak to stick.

What’s more, they’re undermined by the AUMF’s gratuitous overstatement of presidential power: “The President has authority under the Constitution to use force in order to defend the national security interests of the United States.”

Wrong. The Constitution gives him the power to “repel sudden attacks” against the U.S., not launch them whenever he imagines they’ll promote our “national security interests.” That language practically invites Obama to ignore the limits and wage a wider war.

“Parliament has spoken,” UK Foreign Secretary William Hague said after the House of Commons rejected airstrikes.

Meanwhile, Secretary of State John Kerry continues to insist that “the president has the power” to wage war without Congress.

So what happens next? If President Obama loses the vote and launches airstrikes anyway, he should be impeached.

Impeachment was designed as “an essential check,” Hamilton explained, “upon encroachments of the executive.”

Past congresses have missed plenty of opportunities to use that power as intended, but it’s never too late to start.

Back in 1974, Rep. John Conyers, D‑Mich., drafted an article of impeachment based on President Nixon’s secret bombing of Cambodia, carried out in “derogation of the power of the Congress to declare war.”

That charge didn’t make it into the final articles, but it should have. As war powers scholar John Hart Ely once put it:

“I’d have impeached him for it. Surely it would have been a more worthy ground than the combination of a third-rate burglary and a style the stylish couldn’t stomach.”

True, other presidents have gotten away with unauthorized war-making, including most recently, Obama himself, with the 2011 “kinetic military action” in Libya.

That’s an excuse that won’t get you out of a speeding ticket, and it shouldn’t hold for a grave violation of constitutional principle.

I doubt it will come to that, however. Last week, as the intensity of public opposition became clear, the White House started signaling its reluctance to go it alone.

On Friday, deputy national security adviser Tony Blinken said it’s “neither [Obama’s] desire, nor his intention to use that authority absent Congress backing him”; “I’m not itching for military action,” Obama echoed.

Privately, White House aides say that if the vote fails, bombing is “almost unthinkable.”

If, after Congress has spoken, the president holds his fire, the less thoughtful members of the political class will bray that he’s weak.

Maureen Dowd got an early start with a contemptuous column in Sunday’s Times: “When it came time to act as commander-in-chief, he choked,” reverting to “Barry, editor of the Harvard Law Review.”

It’s a measure of how depraved our constitutional culture has become that even members of Congress, like Rep. Peter King, R‑New York, condemn the president for “weakness” because he asked for a vote.

But Americans who care about the rule of law should take a broader view. Whatever Obama’s reasons, this once, he’ll have done the right thing and, perhaps, made it more difficult for future presidents to do wrong.