* Supreme Court ruling expected before 2012 U.S. elections
* Important legal, political, financial implications
* Administration wants to put challenges to rest (Adds background, details)
By James Vicini
WASHINGTON, Sept 28 (Reuters) - The Obama administration on Wednesday asked the U.S. Supreme Court to back the centerpiece of Barack Obama's sweeping healthcare overhaul -- the requirement that all Americans have health insurance.
The appeal was largely expected as a high court ruling against the law could be a fatal blow to the president's signature domestic policy achievement and could have major implications for his re-election bid.
The same day the administration filed its appeal, 26 states and a major business group urged the justices to strike down the entire law, which would have a far-reaching impact on future healthcare coverage for Americans and company costs.
The case is likely to be heard and decided in the Supreme Court's upcoming term that begins next week and lasts through June 2012. A ruling is likely in the midst of the campaign for the November 2012 elections.
The administration and the opponents of the law called for a quick ruling by the high court to resolve uncertainty affecting the federal government, states and companies about the law's key provisions that are taking effect.
The 26 states and National Federation of Independent Business argued in their appeals the entire law should be invalidated because Congress exceeded its powers requiring that Americans buy health insurance or face a penalty.
The Obama administration filed its own appeal in which the Justice Department argued the so-called individual mandate, due to take effect in 2014, was constitutional and said the issue was appropriate for Supreme Court review.
"Throughout history, there have been similar challenges to other landmark legislation such as the Social Security Act, the Civil Rights Act and the Voting Rights Act, and all of those challenges failed," the Justice Department said.
"We believe the challenges to Affordable Care Act ... will also ultimately fail and that the Supreme Court will uphold the law," the department said in a statement.
White House adviser Stephanie Cutter said the administration asked the Supreme Court to hear the case "so that we can put these challenges to rest and continue moving forward implementing the law to lower the cost of health care and make it more secure for all Americans."
At issue was a ruling by a U.S. appeals court in Atlanta in August that declared unconstitutional the individual insurance requirement but refused to strike down the entire law.
That decision conflicted with rulings by other appeals courts that have upheld the law or have rejected legal challenges, including a lawsuit by the state of Virginia that was dismissed earlier this month on procedural grounds. [ID:nN1E78710J]
The law, passed by Congress and signed by Obama in 2010 after a bruising political battle, is expected to be a major issue in the 2012 elections as Obama seeks another four-year term. Republican presidential candidates oppose it and Republicans in Congress have pushed to repeal the law.
Obama, a Democrat, has championed the law as a major accomplishment of his presidency and as a way to try to slow soaring healthcare costs while expanding health insurance coverage to the more than 30 million Americans without it.
The Supreme Court long has been expected to have the final word on the law's constitutionality. The dispute has important legal, political and financial implications for companies in the healthcare field.
Florida Attorney General Pam Bondi said the states sought Supreme Court review of their lawsuit.
"This healthcare law is an affront on Americans' individual liberty and we will not allow the federal government to violate our constitutional rights," she said.
Legal experts have said the nine-member Supreme Court, with a conservative majority and four liberals, most likely will be closely divided on whether the individual mandate requiring insurance purchases exceeded the power of Congress.
The Obama administration earlier this week said it decided against asking the full U.S. Appeals Court for the 11th Circuit to review the August ruling by a three-judge panel of the court that found the insurance requirement unconstitutional.
That decision cleared the way for the administration to go to the Supreme Court.
The states in their appeal also argued the law's expansion of Medicaid, a federal-state partnership that provides health care to low-income Americans, was unconstitutionally coercive, forced upon the states.
A senior Justice Department official told reporters that political considerations played no role in moving for Supreme Court review. The official said it was important to get a ruling soon so planning for the far-reaching law can proceed. (Additional reporting by Jeremy Pelofsky and Karen Pierog in Chicago; Editing by Mary Milliken and Bill Trott)