Two Democratic senators are demanding more transparency about premium increases from health insurers and warning them against blaming higher rates on a newly passed reform law.

Senate Finance Committee Chairman Max Baucus of Montana and Commerce Committee Chairman John Rockefeller of West Virginia said they sent a letter to the five largest health insurers by enrollment registering their concerns over increases for next year.

The letters were sent to WellPoint Inc, UnitedHealth Group Inc, Aetna Inc, Health Care Services Corp and Cigna Corp.

I want health insurance companies to be transparent and honest when increasing premiums -- and health care reform is simply not to blame, Rockefeller said in a statement.

The new health care reform law, championed by President Barack Obama and his fellow Democrats, is likely to be a political issue as mid-term elections approach in November.

The trade organization for health insurers said several factors are responsible for rising premiums, including an escalation in use of expensive procedures and tests and new requirements for benefits not previously covered by health plans.

Health plans will continue to do everything they can to implement the new law in a way that minimizes disruption and keeps coverage as affordable as possible for individuals, families and employers, Robert Zirkelbach, spokesman for America's Health Insurance Plans organization, said in a statement.

Political attacks won't do anything to make coverage more affordable for working families and small businesses that are struggling in a slow economy, Zirkelbach said.

In their letter, the senators said estimates for the new coverage and other provisions from the law taking effect this year call for an increase for health insurance premiums of 1 to 2 percent.

Health insurers should be transparent about the assumptions they use to arrive at their premium increases, the senators wrote. If an insurer thinks it can blame the enactment of the Affordable Care Act for its rising premiums, it is surely mistaken.