WASHINGTON – A tea party candidate’s upset victory Tuesday in Delaware’s Republican Senate primary may jeopardize GOP chances to win control of the Senate in November.

They need to gain 10 seats to run the Senate. Most leading prognosticators had said they appeared to be within reach of that until Tuesday. (Analysts say Republicans still have a good chance to gain a majority in the U.S. House, where they need to pick up 39 seats.)

Delaware is not the only Senate race where Republicans will field a tea party candidate against a well-known Democrat in November. Colorado and Nevada face the same scenario, with incumbent Democrats seeking re-election against tea party insurgents who defeated better-known Republican candidates in low-turnout primaries.

But most analysts believed one of the best chances for Republicans to gain a Senate seat was in Delaware, where the GOP establishment’s choice for Senate nominee was Rep. Michael Castle. A former two-term governor, the popular 71-year-old Castle has won 12 statewide elections and routinely attracts many Democratic and independent votes.

In a closed Republican primary, Castle was upset Tuesday by little-known tea party candidate Christine O’Donnell.

As of Sept. 1, Delaware has a total of 621,746 registered voters. Nearly half are registered Democrats, and another 146,000 are independents.

Despite the intense campaign in the small state, the GOP primary attracted only a 32 percent turnout Tuesday — and a passionate, but relatively small, tea party movement won the majority of a light Republican turnout.

Next, however, O’Donnell must face Democrat Chris Coons, executive of New Castle County.

O’Donnell pulled 30,561 votes Tuesday. In Delaware’s 2006 Senate general election, Democrat Tom Carper won — with 170,567 votes.

Closed primaries, in which only registered party members can vote, tend to reflect the “small, intensely held preferences of fringe groups, and compared to the electorate in open races, the small size of the tea party makes it a fringe group,” said Michael Munger, a political science professor at Duke University.

O’Donnell’s victory makes Republican control of the Senate a “lost cause,” Munger said, “unless the depth of anti-Obama anger is bigger than anything we’ve seen.”

In Colorado, Democratic Sen. Michael Bennet faces tea party-backed Ken Buck. The race is rated a toss-up.

In Nevada, polls show former state Assemblywoman Sharron Angle, the tea party insurgent Republican, deadlocked with Senate Majority Leader Harry Reid.

In Alaska, incumbent Republican Lisa Murkowski was upset by tea party-backed Joe Miller. However, Murkowski is considering a write-in bid to make it a three-way race, so the outcome is difficult to gauge at this point.

The Tea Party Movement has yet to demonstrate that it can extend its reach to a broad constituency. It has also split the Republican Party.

In light of all this, can Republicans pick up 10 Senate seats in November?

Polls show that four incumbent Democrats are locked in tight races for re-election — in Arkansas, Wisconsin, California and Washington.

New Democratic Senate candidates also face tough struggles to hold seats in five states where incumbent Democrats are leaving — Pennsylvania, Illinois, Connecticut, West Virginia and Indiana. That’s nine Democratic seats in play; if the GOP claimed them all, they’d still be one short.

Republicans are expected to pick up the North Dakota seat held by retiring Sen. Byron Dorgan, a Democrat. That could be 10.

However, Republicans face tight races to retain at least five seats they now hold — in Florida, Kentucky, Missouri, New Hampshire and Ohio. Lose any one and even a sweep of the endangered Democratic-held seats would leave the GOP short.

Sen. Lindsey Graham, R-S.C., said that Republicans shouldn’t get overconfident. “If you think that the Republican Party in and of itself has come back in good standing, that’s not the message,” he said. “The message is they want us to check the Democrats.”