House Republicans are demanding more details about the U.S. Census Bureau's decision to change its questions about health insurance in a way that makes it hard to track the impact of President Obama's health care law.

Reps. Darrell Issa, a California Republican who chairs the House Oversight and Government Reform Committee, and Rep. Blake Farenthold, a Texas Republican who chairs a subcommittee that oversees the census, questioned the timing of the revision and why officials at the White House and Health and Human Services Department weighed in and approved the changes.

“Numerous experts from across the political spectrum claim that the Census' new measure will limit the effectiveness of the survey to measure the effects the Affordable Care Act has had on the number of people with health insurance over time,” they wrote in a letter to U.S. Census Bureau Director John Thompson.

The changes, first reported by the New York Times, are taking place just as the American public is weighing Obamacare's impact and trying to figure out the net number of people who have gained insurance versus those who have had plans canceled in the wake of the new law.

A test run with the new questionnaire produced lower estimates of the uninsured than in previous years, which could result in overestimates of the Affordable Care Act's impact on increasing the numbers of those with insurance nationwide.

Thompson on Wednesday defended the timing of the new questions, saying they are based on 14 years of research and two national tests conducted in 2010 and 2013.

The change was announced in September and “implemented because the evidence showed that reengineering the questions provides demonstrably more accurate results,” he said. “The Census Bureau only implements changes in survey methodology based on research, testing and evidence presented for peer review.”

The revised questions were put in the Census Bureau's Current Population Survey starting in February and will cover 2013. He said the new questionnaire will “provide a more accurate baseline for assessments of changes in insurance coverage, including that of the Affordable Care Act.”

Issa and Farenthold, though, said the changes could have a dramatic effect on the way Americans interpret the effectiveness of Obama's signature domestic initiative.

“A two-percent adjustment in the nationwide uninsured rate would represent a change in status for six million Americans and could be used in misleading arguments about the coverage impact of the Affordable Care Act,” they wrote.

They also questioned why the Census Bureau did not inform their committee before implementing the new questions until after the report appeared in the press.

The pair requested all drafts, whether approved or not, and the final text of the revised health insurance coverage-related questions that appear in the February, March and April 2014 versions of the questions.

They also asked for all documents and communications, including emails, in unredacted form, between and among employees of the U.S. Census Bureau, the Department of Commerce, HHS and the Executive Office of the President, from January 1, 2010 through March 1, 2014, regarding or relating to the implementation of the revised health insurance coverage-related questions.

Issa and Farenthold requested the offices hand over the documents by May 1.