Republicans, who currently have 47 of the 100 Senate seats, are seen as having a good shot of winning control of the upper chamber because they are defending far fewer seats in the November election.
Romney, the former governor of Massachusetts, is the clear frontrunner for the party's presidential nomination after victories in the Iowa and New Hampshire nominating contests this month. He can move a big step closer with a win in the January 19 South Carolina primary.
But some supporters of the Tea Party movement and other conservatives distrust Romney, deriding him as a moderate, and they hope to get a few of their candidates into the Senate to serve as a "bulwark" against him or President Barack Obama.
"Conservatives in Nebraska are very concerned about Romney and disappointed that he may be the nominee," said John Arnold of the Lincoln Tea Party. "So the Senate has become much more important."
Democratic Senator Ben Nelson of Nebraska recently said he will not seek reelection in his heavily Republican state. The Republican primary for the seat will pit former state attorney general Don Stenberg - who has some Tea Party backing - against the current state attorney general and "establishment" candidate Jon Bruning.
"The Senate is where we feel Nebraska's conservative values will be best represented," Arnold said.
Republicans would need to win four Democratic-controlled seats and successfully defend all their seats to win the Senate. A net gain of three seats would give Republicans control if Romney or another Republican won the White House, as the sitting vice president breaks all tie votes in the Senate.
While Republicans are expected to lose some seats in the House of Representatives, they currently are expected to retain their majority. If the party gains the Senate then even a few conservative senators could have an outsized impact on the 100-member chamber and U.S. politics.
Last year, conservative freshmen congressmen who won seats in the 2010 midterm elections with Tea Party support pushed the Republicans and Washington further to the right.
Apart from running candidates in battleground states, conservatives are also aiming to unseat Republican incumbents they deem too moderate in solid Republican states. Six-term Senators Orrin Hatch of Utah and Richard Lugar of Indiana face primary challenges, a remarkable turn for a party once renowned for its loyalty and unity.
And in Texas, the primary race for the seat being vacated by Republican Senator Kay Bailey Hutchinson is expected to be a battle between state Lieutenant Governor David Dewhurst, seen as the establishment candidate, and former state solicitor general Ted Cruz, who has a lot of Tea Party backing.
"Conservatives won't win every race they want, but they could win enough to double the size of the Senate's conservative contingent," said Russ Walker, vice president of political and grassroots campaigns at FreedomWorks, a Tea Party-aligned group run by former Republican House Majority Leader Dick Armey. "If that happens and Romney wins, he could go down in the history books as America's most conservative president."
"Not because he wants to," he added. "But because he will have to listen to the Senate."
"NEEDLES IN MY EYES"
It is hard to overstate Tea Party dislike for Romney.
"I'd rather stick needles in my eyes than vote for Mitt Romney," said Pete Harring of the MaineReFounders, a Tea Party group.
In more than a hundred interviews Reuters conducted with Tea Party activists since October, anti-Romney sentiment was prevalent.
Conservative activists are suspicious of Romney's changing positions on abortion and gay marriage. They despise the healthcare bill he passed in Massachusetts, saying it is similar to President Obama's own sweeping 2010 reform. And he is seen as the "establishment" pick by conservative insurgents intent on a hostile takeover of the Republican Party.
Over the past few months, conservatives have embraced then abandoned most of Romney's Republican rivals - U.S. Representative Michele Bachmann, former pizza magnate Herman Cain, Texas Governor Rick Perry, former House Speaker Newt Gingrich and former U.S. Senator Rick Santorum.
Representative Ron Paul of Texas, perhaps Romney's biggest challenger, is not seen going the distance and former Utah governor Jon Huntsman will struggle to improve on the third place he won in the New Hampshire primary.
None of them have united or ignited the conservative base.
"The sad fact is there are no top-tier conservative candidates this year," said Richard Viguerie, author of "Conservatives Betrayed: How George W. Bush and Other Big Government Republicans Hijacked the Conservative Cause."
Activists like Rachel Delgado of the Galveston Tea Party in Texas say they have "not given up" on an alternative to Romney.
"I don't want a candidate picked by investment bankers," she said. "I want a nominee who reflects the will of the people rather than the will of JP Morgan Chase," a reference to Romney's days at private equity firm Bain Capital.
Others like Chris Littleton of the Ohio Liberty Council, a Tea Party umbrella group, are looking beyond the presidential race.
"We don't like him (Romney) and we don't want him," he said. "But he looks like he's the inevitable nominee."
"The Senate is the difference maker and can serve as a bulwark" against either Obama or Romney.
SLIGHT REPUBLICAN EDGE
Taken at face value, 2012 favors Republican chances of taking the Senate. Of the 33 Senate seats up for reelection in 2012, 23 are currently held by Democrats.
Seven of the nine seats listed by The Cook Political Report, an influential political forecaster, as a "toss up," are held by Democrats - including Virginia, North Dakota, New Mexico and Wisconsin, America's newest battleground state. In all four of those states, the Democratic incumbent is retiring.
"As of now the Republicans have a slight advantage for the Senate," said Larry Sabato, a political analyst at the University of Virginia. "But if the economy strengthens that could change. It's really far too early to say."
Sabato said the main question for the Tea Party is "how much they have truly learned" from the failed Senate candidacies in 2010 of conservatives like Christine O'Donnell in Delaware.
Much may depend on who wins the Republican nomination and general election. The Cook Political Report's Jennifer Duffy said that Michigan's U.S. Senate race, for instance, could turn into a "toss up" from "likely Democrat" if Romney is the nominee. Romney's father, George Romney, was the governor there.
While activists like Brenda Roames of The Greenwich Tea Party Patriots of South Jersey say many in the Tea Party will vote for Romney "not with enthusiasm but out of duty" if he's the nominee, most say they will not campaign on his behalf.
"There is no way on earth I'd campaign for Romney," said Tim Dake of the Wisconsin Grandsons of Liberty. "I will put all of my efforts into getting a conservative U.S. senator instead."
Any conservatives elected would join a small cadre of Tea Party favorites already in the Senate, including Jim DeMint of South Carolina, Rand Paul of Kentucky and Florida's Marco Rubio.
If more senators join them and Republicans end up with a majority, conservatives hope they will be able to force their agenda on Romney or Obama. They cite the example of the welfare reform undertaken by Democratic President Bill Clinton in 1996 as an example of what a Republican-controlled Congress can achieve regardless of who is in the White House.
"The Senate gives conservatives their greatest opportunity to have an impact in 2012," said Republican strategist and CivicForumPAC chairman Ford O'Connell. "And the Senate is doubly important because no one should underestimate President Obama, he's the best campaigner I've ever seen."
"Once the Republican presidential nomination process is over," he added, "the real story is going to be the Senate."
(Editing by Alistair Bell and Paul Simao)
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