Senate Judiciary Committee Chairman Chuck Grassley (R-IA) talks with reporters as he heads for a meeting at the Capitol October 02, 2018 in Washington, DC.

Senate Finance Committee Chairman Chuck Grassley doesn't believe a lawsuit challenging the Affordable Care Act's constitutionality will succeed, the Iowa Republican told the Des Moines Register on Thursday.

"I don't believe the courts are going to strike it down," Grassley said, adding that Republicans don't have to work on a replacement for the 2010 health-care law because he said it is unclear what the courts will decide to do.

The Affordable Care Act, or Obamacare, is once again in jeopardy after President Donald Trump reignited debate over his predecessor's health-care law in March when his administration decided to support a federal judge's ruling that Obamacare is unconstitutional.

In December, U.S. District Judge Reed O'Connor ruled the health-care law unconstitutional after 20 Republican-led states filed a suit against it. O'Connor said that without the law's individual mandate — a tax penalty on anyone who did not purchase health insurance — Obamacare could not stand.

The Trump administration reduced the individual mandate to $0 in the 2017 Tax Cuts and Jobs Act.

O'Connor's ruling awaits deliberation in the U.S. Court of Appeals for the 5th Circuit in New Orleans.

Trump has repeatedly criticized Obamacare and pledged to repeal it, but Republicans have yet to propose a health-care policy to replace it that could pass muster in Congress. Trump said in a late-March tweet that the "Republican Party will become the Party of Great HealthCare," but the president was forced to abandon the cause when his party expressed little desire to revisit the health-care debate before the 2020 election.

Grassley, who is one of the many Republicans who oppose President Barack Obama's 2010 health care law, said Thursday that he expects the Supreme Court will uphold Obamacare if the case is brought before them.

He told the Register he "would be very doubtful" that Chief Justice John Roberts would "be changing his mind" on the law. Roberts was the deciding vote that upheld Obamacare's constitutionality when the law was deliberated by the Supreme Court in 2012.

Obamacare, which sought to expand access to insurance and increase consumer protections, includes provisions that expand Medicaid in most states, allow children to remain on their parents' insurance plans until age 26 and protect those with pre-existing conditions.

The health-care law captured the ire of conservatives when it passed in 2010. Republicans have vowed to do away with Obamacare ever since, but the GOP failed to repeal the law in 2017 when they controlled Congress and the White House. Lawmakers on both sides of the aisle believe Republicans' failed efforts to scrap the law handed Democrats a majority in the House of Representatives in last year's midterm elections.

Senate Majority Leader Mitch McConnell said in March that the Senate would wait until Republicans gained full control of Congress before considering a replacement plan. The GOP, which holds a majority in the Senate, needs 21 more seats in the House to win a majority in the next election.

Grassley, who has been working on a bipartisan basis to tackle high drug prices in the U.S., also praised a bill he co-sponsored that protects people with pre-existing conditions. However, Grassley told the Register that the bill, which was unveiled this week, would not be debated unless Obamacare was repealed.

Though Grassley doubts Obamacare will be ruled unconstitutional, he claimed replacement legislation could "easily" be created.

Grassley's office confirmed the Register's reporting, but did not immediately respond to CNBC's request for additional comment.

Read the Des Moines Register's full report here.