WASHINGTON (MarketWatch) — The Republican Party used to be a center-right party, conservative but mainstream. Now it's the home of extreme views.

Just how extreme have Republican leaders become? Here are four things mainstream Washington Republicans used to believe in, but no longer do.

Taxation: The Republican Party has long favored low tax rates as a way to encourage economic efficiency, but its leaders have always recognized that some taxation is necessary and good. Under the old Republican philosophy, the purpose of taxation is to raise the revenues needed by the government. They believed, in theory, that the government shouldn’t spend money it didn’t have, so sufficient revenues were needed.

Ronald Reagan raised taxes 12 times. Reuters

Ronald Reagan cut taxes dramatically in his first year in office, but when the deficits rose, he smartly agreed to raise taxes 12 times, including a broad tax-reform bill that eliminated many loopholes and removed many provisions of the tax code that distorted economic incentives.

Today’s Republican Party has abandoned the principle that some taxation is necessary. The party has fully bought into the “starve-the-beast” rhetoric espoused by Grover Norquist and other anti-tax zealots. It has adopted a rigid libertarian philosophy that equates taxation to tyranny, which argues that the government has no right to your money. Taken to its logical conclusion, this would mean all taxes are immoral.

For today’s Republicans, the proper level of taxation is no longer a question of paying for needed services in the most efficient manner, but is strictly a moral and political matter. The proper level is zero, and only political obstacles stand in the way of achieving that goal.

The current Republican Party pretends to care about deficits, but it clearly doesn’t. It refuses to even consider raising taxes to reduce the deficit, even by closing inefficient loopholes that distort the economy. The Republican-controlled House passed the Ryan budget plan, which cuts taxes and spending by trillions of dollars but does almost nothing to reduce the deficit over the next 10 years

A strong presidency: Republicans have long favored a strong presidency, especially in foreign affairs. Under the administration of George W. Bush, high officials even promoted an extreme theory of a “unitary executive” that claimed that the president was largely above the law, and could do pretty much what he wanted, including ignoring national and international laws banning torture and unlawful imprisonment.

That’s a far cry from today, when Republicans in the House rushed to approve a resolution to condemn President Barack Obama’s involvement in Libya’s civil war, saying he didn’t consult with Congress enough. That was the same complaint some Republicans had about Democratic President Bill Clinton’s decision to fight in Kosovo.

The Libyan crisis put the Republicans in a terrible jam. Generally, they like a good fight, especially against a cartoonish bully like Moammar Gadhafi. When the rebellion first broke out, many Republicans demanded that the U.S. get involved in the civil war. But then Obama ordered a no-fly zone over Libya, just as Republicans had demanded. Nothing infuriates Republicans more than when Obama agrees with them.

It seems as if the Republicans do have a consistent principle when it comes to the president’s war-making powers: Republican presidents are the unquestioned commander-in-chief of the armed services and may start wars wherever they please, but Democratic presidents aren’t even qualified to order lunch, much less boss the troops around.

Protecting our planet: Republicans used to be conservatives in the truest sense of the word. They favored sensible laws to conserve the water, air and land. The Environmental Protection Agency was created by Republican President Richard Nixon.

For years, Republicans favored market-based solutions to environmental problems, rather than relying on government regulations and bureaucrats. For instance, many Republicans, including John McCain, Lindsey Graham, Mark Kirk, Tim Pawlenty and Mitt Romney, favored a plan to give businesses an incentive to reduce greenhouse gases by giving them the right to sell their rights to emit carbon dioxide — what’s known as the cap-and-trade plan to combat climate change.

Cap and trade is a Republican idea through and through; it was modeled on the successful cap-and-trade acid-rain program initiated by the White House under Republican President George H.W. Bush in 1990. Read the Political History of Cap and Trade.

Today’s Republicans reject almost all government actions to protect the environment, saying they kill jobs. They want to drill for oil and gas everywhere, they want the EPA to stop regulating pollution, and they categorically reject all scientific evidence for climate change. They are so dead-set against conservation that they voted to kill a regulation (approved under Republican President George W. Bush) requiring that Americans use light bulbs that actually emit more light than heat.

Universal health coverage: It’s no secret that today’s Republican Party loathes the idea of universal health-care coverage. It believes requiring Americans to buy coverage is unconstitutional and immoral. One of its most cherished goals is to repeal the Affordable Care Act.

But did you know that the Affordable Care Act — derisively known as ObamaCare — is essentially a Republican plan, modeled after a 1993 Republican health-care bill and the Massachusetts state law signed by Mitt Romney?

The ambition of the 1993 bill introduced by Rhode Island Republican John Chafee was to provide universal coverage, with all Americans required to buy insurance. The bill would have subsidized coverage for those with low incomes. It would have prohibited denial of coverage based on pre-existing conditions. And it would have fined employers who did not offer a qualified health-care plan. Sound familiar?

It was sponsored by 19 Republican senators, including Bob Dole, Pete Domenici, Chuck Grassley, Orrin Hatch, Bob Bennett, Dick Lugar, Alan Simpson and Ted Stephens.

You may not recognize all those names, but you should know this group was the Republican establishment in 1993. And that Republican establishment was far to the left of today’s Republican Party.