WASHINGTON — As a Republican congressman who helped impeach President Clinton in 1998, Bob Barr irritated the hell out of Democrats.

Now as a Libertarian seeking to run for president on his new party's ticket, Barr could balance things out. He threatens to hurt the Republicans by siphoning off conservative support from Sen. John McCain of Arizona, the presumptive Republican presidential nominee.

Barr, 59, formally launched his campaign Monday for the Libertarian Party presidential nomination with a vow to slash the size and power of the federal government, restore civil liberties curbed since 2001 and pull back U.S. troops from abroad, both in Iraq and at bases around the world.

He insisted that he didn't consider how his candidacy might affect the major-party candidates, but he noted that several Republican friends had called to urge him not to launch the third-party run. "We would prefer it if you don't run," Barr quoted them. "It would upset the apple cart."

And while he issued a vague criticism of Sen. Barack Obama of Illinois, the front- runner for the Democratic Party's nomination, Barr saved his most pointed criticisms for the Republican Party he left in 2006, a president who he said abandoned fundamental conservative principles such as civil liberties, and McCain, whom he dismissed as anything but conservative.

"If Senator McCain ... does not succeed in winning the presidency, it will not be because of Bob Barr, not because of Senator Obama," Barr said.

"It will be because Senator McCain and his party did not present a vision, an agenda, platform and a series of programs that actually resonated positively with the American people. It also may be because their candidate did not resonate with the American people."

Asked specifically why he objected to McCain's candidacy, Barr paused and said, "How long do we have?"

He said his first complaint was the McCain-Feingold campaign-finance law, which conservatives see as an unconstitutional regulation of political speech. Another, Barr said, is that McCain doesn't go far enough in seeking to rein in the federal government.

He said that McCain's promise to stop pork-barrel spending was a fine start, but just a token. "There really is not a great deal of substance there in terms of a commitment ... to cutting the size of government," Barr said.

While McCain has worked to shore up the conservative base of the Republican Party, he still faces criticism from such powerful voices as radio talk show host Rush Limbaugh and protest votes from as many as one out of four primary voters in recent contests long after he clinched the nomination.

Also, while Rep. Ron Paul, R-Texas, has ruled out a return run as the Libertarian Party candidate — which he was in 1988_ as a presidential contender he continues to draw deep support from Libertarians within the party, suggesting a deep reservoir of support for the Libertarian approach.

Barr's candidacy could draw disaffected voters from McCain in the same way that independent Ralph Nader may draw disaffected votes from the Democratic nominee.

Barr is considered the front-runner for the Libertarian nomination at a convention to be held in Denver on May 25 that also will feature several rivals, including former Democratic Sen. Mike Gravel of Alaska.

Should he win the nod, he then faces the challenge of getting his name before voters; the party says it's now on the ballot in 28 states and working on the rest.

For more on the Barr campaign, www.bobbarr2008.com

For more on the Libertarian Party, go to www.lp.org