One of the big victories by tea-party Republicans in the debt-ceiling measure signed into law Tuesday was securing a requirement that Congress vote later this year on a balanced budget amendment to the Constitution.

The measure would need a two-thirds vote in each chamber, and then ratification by 38 states, to succeed. And most observers believe passage in the Democratic-controlled Senate is all but impossible.

Enter Sen. Mark Udall, the centrist Democrat from Colorado, who has introduced an amendment proposal and said Tuesday that Democratic leaders have chosen his legislation to be considered in the fall.

President Obama and other senior Democrats have opposed any balanced-budget amendment, but the idea is popular with many voters – particularly independents, who are growing more fiscally conservative.

Udall is up for reelection in 2014. Many of his Democratic co-sponsors – including Sens. Claire McCaskill (Mo.), Joe Manchin (W. Va.), Bill Nelson (Fla.) and Ben Nelson (Neb.) – are running this year and need support from centrists.

Republicans in the Senate will likely rally around their own proposal, sponsored by Utah Sens. Orrin Hatch and Mike Lee, which would limit spending to 18 percent of GDP and require congressional supermajorities to raise taxes.

Both measures would require that the president submit and Congress pass balanced budgets. And both provide for exemptions in wartime.

But Udall’s amendment has a couple of provisions that might win over some Democrats. It creates a “Social Security lockbox” (with apologies to Al Gore and his Saturday Night Live impersonator) that his office says would “protect the revenue and outlays of Social Security from any balanced budget requirement.” And it prohibits Congress from providing income tax breaks for people earning over $1 million a year unless the country is enjoying budget surpluses.

“What I’m proposing is the most responsible, thoughtful, and workable balanced budget amendment,” Udall said.

The senator said his proposal marks the first time since 1997 that the Senate will consider a Democratic-sponsored balanced budget amendment. That year, the measure failed by one vote.