WASHINGTON — Today is the last big primary election day this year, and it could be the biggest test yet of tea party influence, as the conservative grass-roots movement appears within striking distance of denying veteran moderate U.S. Rep. Michael Castle the Republican U.S. Senate nomination in Delaware.

So far this year, tea party activists have turned voter anger against Washington into a purge of Republicans they see as too cozy with Democrats and toppled GOP establishment Senate favorites around the country.

The insurgents are on the move again in Delaware. Defeating Castle would be their biggest coup yet, a loss that would rock the political world seven weeks before November’s congressional and gubernatorial elections.

Castle has served Delaware since 1966 as a state legislator, lieutenant governor, governor and, since 1993, the state’s only congressman, usually winning his dozen statewide races by huge margins. The state Republican Party establishment is aggressively boosting his Senate candidacy to fill the seat held until 2009 by Vice President Joe Biden and since then by Democrat Ted Kaufman.

A poll last week found Castle slightly behind media consultant Christine O’Donnell, a perennial candidate who was barely known a month ago and whose personal financial record is a patchwork of debts and delinquent taxes.

O’Donnell has fired up the conservative base with support from Sarah Palin, the California-based Tea Party Express, which has pledged $250,000 to her campaign, and Sen. Jim DeMint, R-S.C.

A Castle loss would be arguably the biggest upset yet, because his centrist record and genial style have made him one of the most popular figures for decades in Delaware, a tiny state where he’s known personally in every hamlet.

But O’Donnell has momentum.

“There’s some evidence (the race) is shifting, that Castle is in trouble and the race is a dead heat,” said Joseph Pika, a professor of political science at the University of Delaware.

He said that O’Donnell’s backers, “fanned by the national voices who have chimed in, are the more motivated to vote and could surprise everyone.”

Castle and the GOP establishment are battling back. On Monday, the state GOP sent reporters a list of what it called “more troubling stories about O’Donnell’s past.”

Earlier this year, the Internal Revenue Service placed a lien against O’Donnell for not paying taxes; she says the IRS later conceded that it had made an error. Dean Patterson, an IRS spokesman, said Monday, “We are unable to discuss individual tax matters.”

Meanwhile, in New Hampshire, former Attorney General Kelly Ayotte had been a strong favorite for the GOP Senate nomination, but polls now show lawyer Ovide Lamontagne gaining; he’s attracted some tea party support. While Ayotte is seen as the establishment choice, she’s also won backing from Palin, who calls her “one tough ‘Granite Grizzly.”‘

Lamontagne is appealing as an outsider, said Andrew Smith, the director of the University of New Hampshire Survey Center. “Lamontagne looks like Ronald Reagan. He exudes enthusiasm,” Smith said. DeMint also backs him.