House Republicans don't have the power to stop Obamacare. But they do have the power to investigate it.

Recent weeks have seen the meltdown of the Obamacare national online marketplace, reported to have cost between $400 million and $600 million so far. There are also indications the administration knew serious problems were coming and hid them from lawmakers who have a responsibility to oversee the program.

The episode has prompted a lot of questions on Capitol Hill. Just how many people have tried to purchase coverage on the exchanges? How many have succeeded? Is the level of interest sufficient for Obamacare to reach its goal of seven million enrollees? Why is the administration being so secretive about it?

Also, what about the security of Americans' confidential health and financial information? Does the struggling system have adequate protections for that?

And once the administration finally gets its website working, will millions of Americans experience sticker shock, discovering that they will have to pay higher premiums and deductibles for coverage? What were the administration's in-house estimates on that?

As House Republicans see it, there is much to talk about. Last Thursday, Rep. Fred Upton, chairman of the House Energy and Commerce Committee, sent letters to Health and Human Services Secretary Kathleen Sebelius, as well as some major Obamacare contractors, wanting to know why HHS officials were painting a rosy picture of the exchanges just weeks before it all came crashing down.

"Staff from your agency who briefed committee staff in August 2013 explained that testing of the [exchanges] was proceeding on schedule and did not identify any problems like the ones now being experienced on HealthCare.gov," Upton told Sebelius. In addition, Upton said a top Obamacare official told the committee on Sept. 19 that consumers would have immediate and full access to Obamacare's programs, "and they will be able to choose a plan and get enrolled in coverage beginning Oct. 1."

That certainly didn't happen. Citing what he calls "a host of broken promises" from the Obama administration, Upton is preparing to call those officials back for more testimony. "We want to look at the rollout, and what they said this summer," Upton told me, "when they absolutely verified that everything was fine and dandy."

In addition to testimony, Upton wants internal documents relating to the exchange's design and testing, plus documents from outside experts involved.

And there are still more questions. How did the Obama team select the contractors involved in the (so far, disastrous) rollout? The Washington Examiner's Richard Pollock has reported that federal officials relied on just one company to design the system. "Rather than open the contracting process to a competitive public solicitation with multiple bidders," Pollock reported, "officials in the Department of Health and Human Services' Centers for Medicare and Medicaid accepted a sole bidder, CGI Federal, the U.S. subsidiary of a Canadian company with an uneven record of IT pricing and contract performance."

That is a classic subject for congressional investigation. And so is the subject of cost: Just how much has the administration spent on the exchanges so far versus its original estimates, and how much will it cost to fix the system now? And what about the administration's story that the exchanges' early problems were due simply to an enormous amount of traffic from people wanting to sign up? Where did that come from?

In addition to Upton, Rep. Darrell Issa, chairman of the House Committee on Oversight and Government Reform, and Sen. Lamar Alexander, the ranking Republican on the Senate Committee on Health, Education, Labor and Pensions, are seeking answers from the administration. In a letter to Sebelius on Thursday, the lawmakers demanded a wide range of information from HHS, "for us to better determine whether any corrective legislative actions are necessary."

The number of Obamacare investigations on Capitol Hill is likely to grow in coming weeks. There's no denying the probes will have a political element, as Republican chairmen in the House lead the charge. And if Obamacare's problems continue, and perhaps expand, the situation could be politically advantageous for the GOP. No one should be surprised if a White House on the defensive accuses Republicans of playing politics.

But the fact is, the investigations are necessary and appropriate; Obamacare is a massive, and massively expensive, federal undertaking that could bring about major changes in the lives of millions of Americans. It must have congressional supervision.

"We're going to be pursuing this with a lot of vigor," said Upton. "This issue is not going away."