Something unexpected happened to Barack Obama as he slipped toward the lame duck inertia that stifles second term presidencies.

He transformed himself into a power hungry, Constitution-busting, imperial president who will simply ignore Congress when it won't give him what he wants.

That, at least, is the line from Republicans fuming at Obama's flexing of executive power to skip past their refusal to enact his agenda.

Such is the political estrangement in Washington, that House Speaker John Boehner has made an unusual threat to take Obama to court, alleging abuse of power.

Democrats dismiss the move as a stunt -- and it is unlikely Obama is in legal jeopardy.

But any charge sheet would likely accuse the president of breaking the law by changing implementation dates of his health reform Act and of exceeding his powers to set climate and immigration policy.

Claims Obama is running rogue were bolstered last week when the US Supreme Court, by a 9-0 margin, ruled he illegally appointed members to a Labor Relations Board, bypassing the Senate.

A bipartisan rebuke from the Supreme Court is not to be ignored.

But to Republicans, the president is showing no mercy, even as conservative columnists like George Will wail at "egregious" constitutional offenses and the top Senate Republican Mitch McConnell warns the administration only obeys "the laws it likes."

In an extraordinary moment at a jovial reception for PGA golf pros at the White House last week, Boehner took Obama aside and told him that Americans "don't trust him to enforce the law as written" and ruled out a vote on immigration reform -- the president's top remaining domestic priority, this year.

Republicans also oppose Obama's use of the Environmental Protection Agency (EPA) to cut global warming emissions from power plants after climate legislation died on Capitol Hill.

Lawmakers from both parties complain meanwhile the White House did not fully consult them, as the law requires, when it swapped Taliban prisoners to free US soldier Bowe Bergdahl.

- 'Sue me' -

But Obama won't be cowed. In fact, the Republican assault has breathed new definition into an administration that had appeared sullen and exhausted with two-and-a-half years left.

"Middle Class Americans can't wait for Republicans in Congress to do stuff. So sue me," Obama said Tuesday.

"As long as they're doing nothing, I'm not going to apologize for doing something.

Obama endured much mockery over his "pen and phone" strategy of targeted executive action -- a pale imitation of the hope and change of 2008.

But now, he is doubling down, and asked his cabinet for new steps he could take within months, and promises to do all he can to fix the US immigration system himself.

The White House argues that each action is within the president's power to set rules and implement laws that have been passed by Congress.

Legal experts are divided on whether Obama is stretching the constitutional envelope or waging the presidency's age-old struggle for influence with Congress.

George Edwards, one of the foremost scholars of presidential power, said such struggles often occur when the White House and one or both chambers of Congress are in different hands.

"My bottom line here is I don't think there is a crisis in separation of powers but there is certainly a lot of tension," said Edwards, a professor at Texas A & M University.

"I don't see that there are great abuses in this case."

- Superficial -

Though Obama might be enjoying a morale boost, his tactics appear more likely to have a superficial rather than lasting political impact.

His approval ratings are near historic lows around 40 percent. A Quinnipiac University poll Wednesday found him shunned by voters on the economy, health care and foreign policy after struggling to impose himself on a string of global crises.

He is thus no help to Democrats as Republicans eye a Senate takeover in mid-term elections in November.

The Quinnipiac survey found only 38 percent thought America would be worse off had Republican Mitt Romney beaten Obama two years ago, suggesting an erosion in the political position of a president who twice won more than 50 percent of the popular vote.

"Would Mitt have been a better fit? More voters in hindsight say yes," said Quinnipiac's assistant polling director Tim Malloy.

Obama's executive maneuvering is causing some Democrats to distance themselves from their own president.

His EPA move for instance is a hurdle for Democratic candidate Alison Lundergan Grimes, trying to unseat McConnell in coal state Kentucky.

Obama's refusal to endorse the KeyStone XL pipeline to carry oil sands crude from Canada -- under heavy pressure from his environmentalist base, is a millstone for Senator Mary Landrieu in a tough re-election fight in Louisiana where oil is king.

And while he can shape American life by changing administrative rules -- on immigration -- for instance dictating which categories of illegal immigrants are deported -- Obama admits his action is short of transformational.

"Whatever we do administratively is not going to be sufficient to solve a broken immigration system," he said.