WASHINGTON, DC - FEBRUARY 06: U.S. Sen. Mike Lee (R-UT) speaks about the war in Afghanistan during a news conference on Capitol Hill, February 6, 2014 in Washington, DC. The bipartisan group of Senators introduced a resolution that would require congressional approval for any military mission in Afghanistan after 2014. (Photo by Mark Wilson/Getty Images)

By David Lawder

WASHINGTON, Feb 27 (Reuters) - The largest group in the conservative U.S. Tea Party movement celebrated its fifth anniversary on Thursday with a pledge to push Congress further to the right and capture the White House in 2016.

At a rally in a hotel ballroom two blocks from the Capitol, lawmakers allied with the Tea Party Patriots said their movement had matured from a raucous protest against the growth of government to one that would engage with the government's machinery to promote a stronger conservative agenda.

"It's about winning a civil debate, not a civil war," said Senator Mike Lee, a Tea Party Republican from Utah elected in 2010.

At its start, he said the movement was more akin to the 1773 Boston anti-tax protest but said it should move towards the constructive work done by the framers of the Constitution, who met at Philadelphia in 1787.

"It's much more important that rather than just picking a fight, we've got to move forward with something," Lee said. "If we grasp onto an agenda, we will catch the Washington, D.C. establishment off-guard."

Many of the suggestions thrown out by lawmakers on Thursday revolved around repealing President Barack Obama's health care reform law, balancing the federal budget, rolling back regulation viewed as oppressive and replacing the tax system with a flatter, straightforward levy - goals often voiced by mainstream Republicans.

The Tea Party sprang forth as a grassroots movement from the turmoil of the 2008-2009 financial crisis, when the government launched massive bailouts in the financial sector. A February 2009 on-air rant by CNBC commentator Rick Santelli against a mortgage bailout plan from the Obama administration is credited by many with launching the movement.

"We're thinking of having a Chicago Tea Party in July," Santelli said from the floor from the Chicago Mercantile Exchange, adding that mortgage derivative contracts should be dumped into Lake Michigan.

Tea Party groups sprang up all over the country to protest the bailouts and massive stimulus spending, and their growth was subsequently fueled by voters angry about Obamacare health insurance reforms. Their influence swept dozens of conservative candidates into Congress in 2010 elections, shifting control of the House of Representatives to the Republicans.

But the Tea Party Republicans clashed with more moderate, establishment Republicans including House Speaker John Boehner, making even legislation to control budget deficits - a core issue for the movement - difficult to pass without support from Democrats.

A messy budget battle in 2011, a brawl over tax hikes in late 2012 and a 16-day government shutdown in 2013 took their toll and fed perceptions that the groups thrived more on anarchy than on promoting achievable conservative policies.

RENEWED VIGOR

But a controversy over Internal Revenue Service targeting of Tea Party groups seeking tax exempt status, government spying scandals and problems with the launch of Obamacare have fueled a resurgence in the groups ahead of congressional elections this November.

"I'm here to show that we are still around," said Kathleen, a Tea Party member from Toledo Ohio. She declined to provide her last name, saying she was afraid that "the IRS is going to come after me."

Randy Liebo, the Tea Party Patriots state coordinator for Minnesota, said many members felt betrayed by Boehner for allowing a year-long extension of the federal debt limit to pass with mostly Democratic votes.

He said his state has added 10 Tea Party chapters in the past three months but noted that the group no longer is interested in simply complaining: It won't meet unless it has an action plan to pursue.

"In this year's elections, if we can take the Senate, it would stop Obama from proceeding along further with his liberal agenda," Liebo said. "The next thing would be to take the presidency in 2016."

A potential Republican contender for that race, Senator Rand Paul, regaled the rally with outrageous stories of government waste and pledged that his budget plan would end deficits in five years.

Paul and Lee both likened the Tea Party movement's current development stage to the backers of Ronald Reagan in the late 1970s. They had tried unsuccessfully to get him nominated in 1976, but kept at it and swept him into the White House in a landslide in 1980.

Calling Reagan a "leader for the ages," he said a similar shift could happen now if the Tea Party can focus and promote its message passionately.

But the day's biggest standing ovation came for Senator Ted Cruz, the Texas Republican who may be Paul's rival for the party's 2016 nomination.

Cruz, whose rock-star welcome was a stark contrast to the complaints from some senators after he put up procedural hurdles to the recent debt-limit hike, said he was "absolutely convinced we are going to repeal every single word of Obamacare."

"I think the Tea Party is the most exciting political development in decades," Cruz said. "The great thing about the Tea Party (is) it arose from beneath. It's not top-down, which by the way confuses the heck out of politicians." (Editing by Cynthia Osterman)