The deal between Iran and six world powers to freeze part of its nuclear program is finalized and will go into effect Jan. 20, the Obama administration announced Sunday as it fought a push in Congress to pass new sanctions legislation that it said would derail the pact.

The initial agreement with Iran first reached in late November is only a preliminary one that would test Tehran's ability over the next six months to rein in its nuclear program in exchange for the easing of some U.S.-led international sanctions.

Obama and other senior administration officials Sunday said establishing a start date for the deal shows real, verifiable progress in rolling back Iran's nuclear program.

“With today's agreement, we have made concrete progress,” Obama said in a written statement.

He also pressed Congress not to move ahead with a bipartisan sanctions bill that has gained momentum during the first weeks of the new year and reiterated that he would veto any type of new sanctions measure that arrives on his desk.

“Unprecedented sanctions and tough diplomacy helped to bring Iran to the negotiating table, and I'm grateful to our partners in Congress who share our goal of preventing Iran from acquiring a nuclear weapon," he said. "Imposing additional sanctions now will only risk derailing our efforts to resolve this issue peacefully, and I will veto any legislation enacting new sanctions during the negotiation."

But support for a bill co-authored by Sens. Bob Menendez, a Democrat from New Jersey who chairs the Foreign Relations Committee, and Mark Kirk, R-Ill., has increased significantly since December, reaching 59 co-sponsors. Senate aides say a veto-proof majority - senators numbering well into the 70s - have pledged to support the bill if it comes to the Senate floor for a vote.

Menendez and Kirk have argued that the measure would not impact the negotiations because it gives Obama more than a year to continue diplomacy before any new sanctions would kick in.

Reacting to news of the deal Sunday, Kirk said the Obama administration is rewarding Iran with billions in economic benefits while requiring far too little in return.

"Beginning January 20th, the administration will give the world's leading state sponsor of terrorism billions of dollars while allowing the mullahs to keep their illicit nuclear infrastructure in place," he said. " I am worried the administration's policies will either lead to Iranian nuclear weapons or Israeli air strikes."

"It's time for the United States Senate to pass common-sense bipartisan legislation, now cosponsored by 59 senators, to ensure this process leads to the peaceful dismantlement of Iran's nuclear program," he added.

Democratic senators, including Sen. Michael Bennet of Colorado, who chairs the Senate Democrats' re-election efforts, are defying Obama's warnings about supporting a new Iran sanctions bill and have signed on over the last week. The blatant Democratic defections reflect lingering frustration over the botched rollout of Obamacare, deep distrust lawmakers have for Iran, as well as concern that U.S. officials won't be able to extract lasting concessions from Tehran.

“A lot of Democrats we're talking to say we have no favors to give Obama right now,” a Senate aide told the Washington Examiner Friday. “And when the [administration] says just give us some slack and help us out on this one – they're not inclined to do that.”

Reports that Iran planned to move forward with developing an advanced nuclear centrifuge that would purify plutonium in a more efficient way also spurred more support for the bill.

“That means this entire [diplomatic] process has not changed their ultimate goal – they're still trying to break out and develop nuclear weapons,” the aide said.

The technical details of the agreement reached Sunday aren't public and they don't prevent Iran from developing a more efficient centrifuge, senior administration officials confirmed to reporters on a conference call. Instead, the newly inked pact outlines how the International Atomic Energy Agency will verify that Iran is keeping its commitments and the timing of different requirements and sanctions relief.

As part of the deal, Iran must render its entire stockpile of 20 percent enriched uranium unusable for further enrichment and limit its ongoing enrichment to 5 percent.

“As this agreement takes effect, we will be extraordinarily vigilant in our verification and monitoring of Iran's actions, an effort that will be led by the [IAEA],” Secretary of State John Kerry said in a statement.

Kerry said the U.S. and its international partners will release a total of $4.2 billion in restricted Iranian assets in regular installments through the six months, with the final installment only becoming available on the last day of the agreement.

House Majority Leader Eric Cantor, a Virginia Republican and strong ally of Israel in Congress, said the agreement only "legitimizes Iran's flagrant violations of multiple U.N. Security Council resolutions calling for the full suspension of its nuclear program"

"Although Congress still needs to investigate key details of this implementation plan, the underlying agreement does nothing to reverse Iran's nuclear program, allows it to continue industrial-scale enrichment of uranium and fails to address critical aspects of Iran's weaponization research," he said.