Even as some of the first pieces of President Obama's health care reform legislation take effect, Republicans in Congress and conservative activist groups are still working to repeal or at least rewrite major sections of the legislation.

Rep. Steve King, Iowa Republican, is circulating a petition that would force an up-or-down vote in the House of Representatives on repealing the vast bulk of the estimated $940 billion, 10-year legislation the Democratic-controlled Congress passed this spring.

The petition had 80 signatures as of Thursday - the same day the White House formally launched HealthCare.gov, which officials described as a "one-stop shopping" place for health insurance. Other parts of the law, including a new 10 percent excise tax on indoor-tanning services and a new stopgap program to cover Americans with pre-existing health conditions who can't get private-sector coverage, also went into effect Thursday...