When President Biden and 10 bipartisan senators reached an infrastructure agreement in June, increased IRS enforcement was a key for how to pay for it.

The idea, which has long been pushed by policymakers on both sides of the aisle, was to staff up the IRS now with the promise that the investment would be paid back in spades as more tax cheats were caught.

But a wave of Republican pushback led to the idea getting nixed. The lead Republican negotiating the deal, Sen. Rob Portman of Ohio, recently confirmed it was off that table.

But in interviews this week, Democrats made clear the IRS provision remains very much alive as part of the their parallel effort to pass a reconciliation bill.

“This is something the president feels strongly about,” Biden’s top economic aide, Brian Deese, said in an interview with Yahoo Finance. This week marks Biden's first six months in office.

‘We need professionals there’

“You look at economists across the board, and they would say this is not only a no-brainer from a policy perspective, but would generate much needed revenue,” Deese said.

In a separate interview with Yahoo Finance, House Transportation Committee Chairman Peter DeFazio (D., Oreg.) pointed to falling IRS staff levels over the last decade. Agency turnover is too high, he said, adding “we need professionals there.”

The funding to go after tax cheats will “make those people pay under existing law what they're supposed to pay,” DeFazio said.

[Read more: America's broken tax system]

The annual tax gap – the difference between what Americans should be paying and what they actually send to the IRS – between 2011 and 2013 was $441 billion, the IRS found. A paper published this year found that the wealthiest 1% of Americans underreport their income by 21%, using complicated tax-dodging schemes the IRS often fails to detect even when they audit returns.

DeFazio, who has his own bill to tackle the tax gap, estimates that the average household pays an additional $3,000 annually in taxes to make up for those taxpayers who aren’t paying all that they owe.

Some Republicans have said they're wary of boosting funding for the IRS because it would give more power to an agency regarded with suspicion by the right following years where they aggressively scrutinized conservative groups over their tax-exempt status.

‘A different tax system than everybody else’

The bipartisan plan had been to spend $40 billion to bolster tax enforcement, which would generate more than $100 billion over the next decade, according to the lawmakers negotiating for the deal. Democrats appear committed to include that funding in their upcoming reconciliation bill and may even try to go further to beef up the IRS.