Mitt Romney needs the tea party’s vigor to ensure the GOP’s conservative base is fired up and works on his behalf this fall. The three-year-old tea party movement needs the Republican Party and its new standard-bearer to champion its causes and get its agenda enacted. And the two sides have never had much affinity for the other.

Yet, the tea party has left a solid imprint on its first GOP presidential convention, from the speeches to the platform to the displays of deep opposition to President Barack Obama. With this backdrop, Romney — hardly the first choice of this group of insurgents — will accept the nomination Thursday.

“They’ve been very successful in leaving their mark at this convention,” said Republican National Committee Vice Chairman Jim Bopp of Indiana. “They are a vital part of our coalition.”

But Romney and the tea party have a difficult past.

The former Massachusetts governor, whose signature accomplishment was passage of a state health care overhaul that Obama’s was modeled after and that the tea party despises, never warmed to the insurgent activists and resisted a full embrace of them.

As the GOP looked for a presidential candidate, the tea party rallied behind just about everyone for at least a time, from Minnesota Rep. Michele Bachmann to Georgia businessman Herman Cain to former House Speaker Newt Gingrich to former Pennsylvania Sen. Rick Santorum.

It’s safe to say the tea party was, at best, resigned to Romney as its nominee when it got a welcome boost as the candidate chose Wisconsin Rep. Paul Ryan — a favorite among this group — as his running mate.

The tea party is more influential within the Republican Party than within the overall voter population.

According to a recent Associated Press-GfK poll, 48 percent of Republicans considered themselves supporters of the tea party movement. However, only 29 percent of all registered voters considered themselves tea party supporters, and less than 10 percent said they strongly support the movement.

If they turn out in force, these voters could be influential in competitive states such as Iowa, Michigan, Ohio and Wisconsin, where some conservatives have had misgivings about whether Romney is conservative enough.

With Ryan on the ticket, there was something for them — and their preferred politicians — to celebrate this week in Tampa.

“This national movement is fueled by what unites us: a love of liberty, a belief in the unlimited potential of free men and women,” Ted Cruz, a tea party-backed U.S. Senate candidate from Texas, declared from the podium this week, igniting cheers from inside the Tampa Bay Times Forum.

While not everyone made such direct overtures to the tea party, its rhetoric of uncompromising spending cuts, strict constitutional adherence and limited government dotted speeches from the GOP’s rising stars to the graybeards.

In attendance are some of the movement’s heroes, including New Hampshire Sen. Kelly Ayotte and Florida Sen. Marco Rubio. Another favorite, Kentucky Sen. Rand Paul, was greeted with cheers of “Liberty!” when he stepped to the podium Wednesday.

“We have nothing to fear except our own unwillingness to defend what is naturally ours, our God-given rights,” Paul said during his speech to the convention, prompting a renewed roar.

The insurgents’ imprint went beyond the rhetoric.

Tea party activists also won changes to the Republican Party’s platform, including nearly a dozen fiscal positions, such as studying returning the United States to the gold standard and requiring a two-thirds majority in Congress to pass a tax increase.

But there was tension behind the scenes when party officials sought to weaken tea party influence.

First, the RNC’s rules committee voted that it could change the party rules in the middle of the four-year cycle — instead of every four years, as has been the practice. This is a particularly sensitive point with the tea party’s rule sticklers.

Another new rule was aimed at limiting the ability of insurgent presidential candidates to amass delegates to future Republican conventions. They require states to award delegates to candidates based on the outcome of primaries and caucuses.

The new rules originally allowed the likely presidential nominee to choose which delegates would represent them at the convention — taking that power from state parties, which prompted sharp opposition from tea party activists. In a concession, party leaders agreed to remove the language, instead saying delegates who support candidates other than the one they are obligated to support shall have their votes nullified.

The change appeased some state party activists who didn’t like the idea of candidates mandating which delegates could attend the national convention. But it didn’t satisfy supporters of Texas Rep. Ron Paul or many tea party activists.

“Most of the tea party activists are really unhappy about this,” said Oregon delegate Russ Walker. “We feel it’s kind of cast a black cloud over the success we had with the platform.”

The tea party can be traced to 2008, when voters started protesting spending.

The group broke through nationally in 2009 as opponents to federal legislation expanding access to health care, and then fueled a wave that carried Republicans to enough victories in the House for control.

Now, many tea partiers say the GOP owes them.

“It would have been nice to see some more of a tea-party presence,” said Nevada Republican James Smack. “We can’t win without them.”

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Associated Press writer Stephen Ohlemacher in Washington contributed to this report.