* Tea Party favorites ahead or even in key races - polls
* Activists eye next Tea Party showdown in Delaware
By John Whitesides
WASHINGTON, Sept 1 (Reuters) - With another win in a U.S. Senate Republican primary, this time in Alaska, the conservative Tea Party movement showed it is more than a political fad and has the staying power to be a significant force in November's elections.
Polls show Tea Party favorites leading or running nearly even with Democratic foes in a handful of high-profile Senate races that could shift the balance of power in Congress -- or at least inject a potent new strain of anti-spending, anti-big government conservatism into the staid Senate.
Republican Joe Miller's win over Senator Lisa Murkowski in the Alaska primary was the movement's latest success. Murkowski conceded the race on Tuesday, becoming the seventh incumbent to lose a congressional primary this year and the most recent Republican to fall under a wave of anti-establishment anger.
The Tea Party's platform of limited government, lower federal spending and opposition to President Barack Obama's agenda has proven a good match with the public's mood of unhappiness with Washington.
"The Tea Party is bringing energy and voters to Republicans, and Democrats can't seem to change the subject," said Steven Schier, a political scientist at Carleton College in Minnesota. "They have generated a ton of excitement and turned out votes."
For weeks, Democrats have cheered the success of conservative Tea Party Senate candidates in states like Nevada, Colorado, Kentucky and Florida, predicting they would falter in November by alienating moderates and energizing Democrats in those critical races.
But polls show Republicans Ken Buck, Rand Paul and Marco Rubio hold slight leads in Colorado, Kentucky and Florida, respectively. Tea Party-favorite Sharron Angle runs only slightly behind Senate Democratic leader Harry Reid in Nevada in one of the year's spotlight races.
"Democrats are doing a great deal of whistling past the graveyard these days," said Republican strategist Rich Galen. "They are looking for something to hang their hats on, but the Tea Party isn't going to be it."
Republicans need to gain 39 seats in the House of Representatives and 10 seats in the Senate to regain control of those chambers and put the brakes on Obama's legislative agenda.
While a Republican House takeover is seen as a growing likelihood, Republicans must run the table of competitive races to capture the Senate -- a difficult, but not impossible task.
The races in Nevada and Kentucky, where Angle and Paul have been uneven and sometimes mistake-prone candidates, could prove to be the toughest tests for Tea Party contenders.
Both candidates have tried to move beyond early gaffes, like Paul's questioning of the Civil Rights Act, and focus on economic concerns that resonate with voters across the spectrum.
"Nevada and Kentucky will be the Tea Party's real proving ground," said Jennifer Duffy, the Senate analyst for the nonpartisan Cook Political Report.
"At least in the Senate, the Tea Party could still be a mixed bag for Republicans," she said. "What if the Republicans pick up eight seats but lose Nevada and Kentucky -- what do you say about the Tea Party then?"
After the win in Alaska, Tea Party activists are already taking aim at their next establishment target -- nine-term Republican Representative Michael Castle of Delaware, who is running for the Senate.
Castle was expected to easily walk into the Republican nomination and win the former seat of Vice President Joseph Biden in November. Biden's appointed replacement, Ted Kaufman, is not seeking re-election.
But Tea Party activists have swung behind Castle's primary challenger Christine O'Donnell. After the results in Alaska few are willing to predict Castle is the certain nominee.
"He saw what happened in Alaska," Galen said of Castle. "He has to take this challenge seriously."
Editing by Vicki Allen