First-term Senators could make up 40 percent of the chamber next year, wielding significant influence over their parties and the institution itself.
The seniority shift comes after two consecutive cycles of Democratic gains, and a 2010 election that is likely to give Republicans a historic number of new Members.
'The clash is going to come when the new crop of Republicans meets the newer classes of Democrats who have absolutely had it with the Republican stalling tactics of the past couple of years,' said one senior Senate Democratic aide who has worked in the chamber for decades.
Many of the Republicans who win this year will have conservative tea party enthusiasts to thank, and they will come to the Senate prepared to stand firm against President Barack Obama's agenda.
By contrast, the 2006 and 2008 classes of Democrats are largely liberal-leaning and have been using their first terms to try to change Senate rules so Republican lawmakers, such as Sens. Tom Coburn (Okla.) and Jim DeMint (S.C.), cannot block Democratic priorities.
Republican Conference Chairman Lamar Alexander (Tenn.) said he hopes the group of incoming GOP Senators will 'diminish the enthusiasm among some of the new Democrats to rip the Senate apart by destroying minority rights' in their push to limit or eliminate filibusters.
But the longtime Senate Democratic aide warned that if Democrats hang onto control of the chamber, junior Members may have even greater incentive to change the rules, especially if 'the newer crop of Republicans brings the place to a [legislative] standstill.'
Republicans are expected to win at least 11 seats, and polls favor the GOP in 10 more races this year, according to Roll Call's current ratings. Eight other races have been rated tossups.
The potential for 21 or more new Republican Senators would rival the 20 Democrats who joined the chamber over the past two election cycles.
Alexander said, 'It'll be a historic number of new Republicans.'
'It's going to form the core of the Republican Senate for the next two decades and form the next generation of leaders,' he said.
The makeover could be so dramatic that Sen. Scott Brown (R-Mass.) could jump from being ranked dead last, or 41st, in party seniority to 33rd, after serving less than a year, Alexander said.
And Alexander argued that the class may not tilt as heavily to the right as many have suggested. Instead, he said Republican freshmen are likely to have 'diverse points of view.'
Still, he predicted Republicans would have near unanimity 'on the debt, limited government and free markets.'
'On our side, we need to make sure we listen to what the voters are saying this year,' Alexander said.
But one veteran Democratic Senator said Alexander and other GOP leaders could be surprised by the desire of new Members to upset the way the chamber operates.
'Most people over the decades have come to the Senate ... and taken their time to integrate into the institution,' the Democratic Senator said. He added that has not been the case for the Democratic classes of 2006 and 2008.
'These new people come to the Senate at a time when the economy is in trouble, jobs are being lost,' the Senator said. 'As a result of that, they have been more aggressive in telling leadership, 'Look, we've got to have an agenda here.''
The Senator added of Republicans: 'I think they're in for some real difficulty. ... If a big class is coming in from the disaffected side of politics, that's going to cause some real trouble.'
Republicans saw a large influx of new Members in 1946 and again in 1980 ' both years in which Republicans won back the majority. In 1946, Republicans welcomed 17 new Members; in 1980, they welcomed 16.
'As it did in 1980, [this election] will make it easier for Republican voices to be heard in controlling the direction of the country,' Alexander said.
Senate Democrats have also seen ideological realignments: in 1932 and in the combination of the elections of 2006 and 2008. Nineteen Democrats helped the party take over the chamber in 1932; in 2006 and 2008, Democrats gained eight and 12 new Members, respectively.
'In some cases, particularly the 1932 election and the 1980 election, you're looking at a time when the party system was undergoing great change,' Senate Associate Historian Betty Koed said in an e-mail.
She said the Democratic takeover of the Senate in 1932 'was the culmination of a trend beginning in the 1920s that changed the make-up of the Democratic party, bringing into its fold African Americans, many immigrant groups, and farmers, who had been primarily Republican before that.'
The Great Depression helped fuel the change that had been under way since 1926, Koed said.
Similarly, the 1980 election was the result of years of changes that benefited the GOP, she said.
'The victory of Ronald Reagan and congressional Republicans in the 1980 election had its roots in the 1960s, when a combination of civil rights legislation, general expansion of government programs, and a growing concern over war issues brought new voters to the Republican Party,' Koed said.
'The 1968 election shifted southern voters from the traditional Democratic stronghold in the South to the 'solid Republican South' of later years, a trend that became a reality with the 1980 election,' she added.
Koed noted that other issues, such as President Jimmy Carter's struggles with the economy and the Iran hostage crisis, as well as Reagan's charisma and political operation, contributed to the effectiveness of GOP campaigns.
Democrats said their only hope, if this year is to be another 1980, is that six years from now will bring another Democratic resurgence, as happened in 1986, when Democrats gained 11 seats.
'The conservative class of Republicans [in 1980] quickly flamed out because they were not temperamentally suited to be in the Senate,' the Democratic aide said. 'The crop that may be coming in could be more conservative than that.'