FOR VOLATILE PRIMARY SEASON, A SUITABLE ENDING Kate Zernike contributed reporting from New York, and Michael D. Shear from Washington. As the long and turbulent primary season of the midterm election campaign drew to a close Monday, the Republican establishment was placing its confidence on hold and bracing for the prospect that voters in yet another state would send a message of defiance to party leaders in Washington. The Senate primary in Delaware on Tuesday was prompting anxiety among party officials, who feared that a victory by Christine O'Donnell, a candidate backed by the Tea Party, could complicate Republican efforts to win control of the Senate. Republican leaders rushed to the aid of Rep. Michael N. Castle, a moderate lawmaker and former governor, as internal party warfare -- including accusations of a death threat -- intensified on the eve of the primary. 'We need to prove to the Republican Party that we need to move it to the right, that we need to move back to good basic values and the Constitution,' said Lynn Brannon, a leading Tea Party activist in Delaware. 'I will have a little bit of regret, but the Republicans need to learn their lesson: that we want things to go back to the right.' It is a fitting end to an unusually volatile seven-month stretch of primaries that demonstrated in state after state that the discontent percolating in the electorate does not discriminate by political party. Voters in seven states will choose nominees in House, Senate and gubernatorial races Tuesday, the final marquee primary day of the election cycle. The Senate contests in Delaware and New Hampshire were being watched closely by Democratic leaders, who believe that divisive purity tests in Republican primaries have improved their chances of retaining their majority. 'Republicans have chosen extremists to be their nominees, and this has changed the political map of the cycle,' said Sen. Robert Menendez of New Jersey, chairman of the Democratic Senatorial Campaign Committee. 'In a year where Republicans want these races to be all about Democrats, Republican nominees who have extreme positions help us make the contrasts we need to make.' Republicans are finishing the primary season in a stronger position than they imagined when the year began, when winning control of the Senate was not included in even the most optimistic projections. But discord inside the party has steadily accumulated and will test Republicans in November and beyond. Seven candidates supported by the National Republican Senatorial Committee have already fallen this year. When Sen. Robert F. Bennett of Utah, a three-term Republican, was defeated before reaching his primary, party leaders in Washington dismissed his loss as a product of the state's quirky political system, where candidates must clear a series of hurdles with party activists before making the ballot. But back in the spring, shortly before his defeat, Bennett detected that trouble was on the horizon for party's establishment members. 'A politician always thinks he's in better shape than he really is,' he said. His defeat was followed by campaigns in Colorado and Connecticut, Florida and Kentucky, Nevada and Alaska, where insurgents knocked off candidates initially favored by the Republican leadership in Washington. Sen. John Cornyn of Texas, chairman of the National Republican Senatorial Committee, has argued that the November elections will be a referendum by voters on the policies of the Obama administration and the Democratic-led Congress. 'Voters see the need for checks and balances,' Cornyn said. The race in Delaware is to fill the seat that Vice President Joe Biden held for 36 years. Republicans recruited Castle, a fixture in state politics, and until recently even many Democrats believed he would be difficult to beat. But when Sen. Lisa Murkowski of Alaska lost her Republican primary last month to a candidate backed by the Tea Party, activists were emboldened and the Delaware contest came to life. O'Donnell, who until this year was a perennial candidate who struggled with her finances and other elements of her personal life, was endorsed by Sarah Palin and Sen. Jim DeMint of South Carolina. But other conservative leaders and groups steered clear of her race, saying that the purity test among Republicans could be taken too far. FreedomWorks, an advocacy group that has helped expand the Tea Party movement and train groups to help with voter turnout, did not support her candidacy. 'It's not wise to elect a philosophically perfect candidate who is not capable of winning the general election,' said Matt Kibbe, president of FreedomWorks. 'We've stayed out of the Delaware primary because we worry that Christine O'Donnell can't win the general election.' The Delaware campaign took an odd turn Monday, as Republican officials disclosed that the party chairman, Tom Ross, had received a death threat that was under investigation by the Justice Department in Washington. He has been outspoken in his support for Castle and his criticism of O'Donnell. The outcome of the Senate primary in New Hampshire was less worrisome to Republican leaders in Washington. The party's top candidate, Kelly Ayotte, a former state attorney general, also won the backing of Palin. She appeared to be locked in a close contest with Ovide Lamontagne, a lawyer and a longtime political figure in the state. The winner of the contest will face Rep. Paul W. Hodes, a Democrat. Republicans must win 10 seats in November to win control of the Senate. There are few ways they can do so without holding the Republican seat in New Hampshire. 