"That's why I will vote for it this week."
Hatch opposed immigration reform in 1986 and was among those who helped defeat a similar bill in 2007. But he says this time is different because of beefed-up border security and tougher penalties for those seeking legal status.
He rejects any claim that the bill offers "amnesty."
In his column, he noted that the pathway to citizenship requires an immigrant to pay a fine, pass background checks, stay current on taxes and wait at least 10 years to get a green card.
"That's hardly amnesty," he said.
His Utah GOP colleague, Sen. Mike Lee, hasn't called it amnesty, either, but he says the legislation is still too easy on lawbreakers.
Lee will oppose the bill when the Senate votes, saying Congress should tackle this issue in pieces, starting with the border and ending with what to do with the undocumented immigrants already in the U.S.
The legislation is expected to pass the Senate. The question is by how much. Hatch and the bill's bipartisan sponsors have sought at least 70 of the 100 votes to put pressure on the GOP-controlled House to take up similar legislation.
Hatch offered a series of amendments, including the back-taxes idea, that he said would lure more Republican support, and he repeatedly vowed to oppose the bill if Democrats didn't accept them.
Supporters of the bill didn't budge on taxes or on Hatch's proposed delay of health-care benefits for immigrants, arguing that calculating back taxes would be unwieldy and that postponing Obamacare coverage for a select group would be unfair.
Hatch has now backed away from his demands but won't drop the back-taxes issue entirely.
"I will work with the House of Representatives to ensure that we continue to improve this bill for American families and taxpayers," he wrote.
Backers of the bill did accept two Hatch ideas that would require the government to follow welfare rules and block unauthorized immigrants from receiving Social Security benefits.
Those amendments are part of what's been dubbed the "border surge," a deal to double the size of the Border Patrol and build 700 miles of fencing, which has attracted more Republican support for the legislation.
Hatch's fingerprints are on other parts of the deal as well.
He helped negotiate new programs allowing companies to temporarily employ high-skilled foreign workers and agricultural laborers. He sponsored a program giving states a portion of the fees from new visas to fund education programs in math and science.
Hatch also pushed to permanently extend a visa program for religious workers. He also helped create a pilot program to track foreign nationals leaving the nation's 10 busiest airports, which, if successful, would expand to other major travel hubs. Finally, he offered an amendment to increase criminal penalties for growing marijuana on federal lands, a practice often employed in Western states by Mexican drug cartels.