Fighting to retain the seat he won in a surprising victory two years ago, Republican Senator Scott Brown was working hard to energize an already enthusiastic group of supporters on Sunday who had crammed inside Boston's historic Faneuil Hall.

"We're going to do this," said Brown, who reflected back on his unlikely Tea Party-backed win in the 2010 special election, when he captured the Massachusetts Senate seat previously held by a Democratic icon, the late Edward Kennedy.

"The energy was unbelievable" in 2010, Brown said. But "let me tell you something. The energy level right now makes that pale."

Brown may be forgiven a little election hyperbole. After all, over at Braintree High School, just outside of Boston, his Democratic opponent, law professor Elizabeth Warren, seemed to be talking from the same campaign script: "I’m excited about this. I feel the energy out here. We’re going to do this."

Energy is one campaign ingredient, of course. But votes are something else, and, come Tuesday, many observers believe that Warren will eke out a victory over the junior senator, whose loss may in some ways symbolize the dashed hopes Republicans had for retaking the Senate and so dominating both houses of Congress.

Most polls also suggest that the Republicans will retain the House of Representatives. But the U.S. Senate is proving to be a harder nut to crack. At stake are not just individual fortunes but — depending on who wins the White House — the prospect of a Republican-controlled Congress that could easily repeal Obamacare and slice through any number of legislative changes.

Slim majority

As things stand, the Democrats hold a slim majority in the Senate, with 53 (including two independents) of the 100 seats. But Republican optimism was not without reason, as the Democrats had more seats of the 33 up for grabs this time and more retirements.

But a number of things happened to thwart Republican ambitions, including the retirement of Maine's Olympia Snowe and the nomination of controversial, usually Tea Party-backed Republicans in what had been considered safe GOP seats.

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The Democrats' hope for Massachusetts Senate seat, Elizabeth Warren, is endorsed by singer James Taylor at an event in Boston last month. (Associated Press)

"It's kind of a mix of bad luck and self-inflicted wounds," Geoffrey Skelley, political analyst at the University of Virginia's Centre for Politics, told CBC News.

By stepping aside, Snowe, a long-serving moderate Republican senator from Maine, made a safe Republican seat vulnerable to Democratic takeover.

But more crucial to Republican fortunes was what took place in the Senate primaries earlier this year, where "we have Tea Party arch-conservative candidates beating out more establishment-type candidates in some key races the Republicans expected would be gimmes,"  Matthew Baum, professor of public policy at Harvard University's John F. Kennedy school of government, said in an interview.

For example, Republicans chose Richard Mourdock over veteran Indiana Senator Richard Lugar and nominated Todd Akin in Missouri — two candidates who gained national notoriety for controversial comments about rape and abortion.

"If [Lugar] doesn’t lose the primary in Indiana, he wins [the state] in a cake walk," Skelley said. "In Missouri, [Democrat]

Claire McCaskill is extremely vulnerable but the person who won the Republican nomination was the least electable. He probably would have won anyway but then he of course made his comments about 'legitimate rape.'

"I mean Romney is going to carry Missouri and Indiana soundly, but the Democrats have a pretty good shot of holding on to Missouri and do have a shot of winning in Indiana" Senate races.

Competitive races

In Virginia, where Republicans had, and maybe still have, a good chance of electoral success, they nominated former senator George Allen, who lost the seat in 2006 for uttering the word "macacca" — a term considered by some to a racial epithet  — to run against former Democratic governor Tim Kaine.

A similar battle is taking place in North Dakota, where Republican Rick Berg is in a tough fight against former attorney general Heidi Heitkamp.

Here in Massachusetts, while Brown hasn’t made any real political mistakes, he could just be a victim of living in a blue state, one that Barack Obama should easily win, despite the fact that Mitt Romney was once its governor.

"This is overwhelmingly a Democratic state," says Baum. "What Warren needed to do … was to remind Democrats and liberal-minded folks that they're liberal-minded folks.

"Come home, basically. That's a much easier argument to sell than Scott Brown's argument which is: 'I’m non-partisan and even if you're not in my party, you can trust me to stand up to my party.' That's a hard sell."

Looking at the bigger picture, Jennifer Duffy, who covers the U.S. Senate for the Cook Political Report, said Democrats should get credit for recruiting strong candidates in certain red states and keeping those races competitive.

"They succeeded in growing their field of opportunities in a way Republicans really didn't but needed to."

Many of these races still could go either way, like one between Arizona Republican Jeff Flake and Democrat Richard Carmona and the perhaps most up-in-the-air Senate race: the battle between Gov. Tommy Thompson and Democratic congresswoman Tammy Baldwin in Wisconsin.

Still, if the Republicans do end up in control of the Senate and the White House, America is likely to see some big changes ahead.

"I think it's highly likely they'd use budget reconciliation rules to pass a great deal of their agenda," Baum said, referring to the congressional procedures that can be implemented to restrict debate and filibusters on bills that have a revenue component.

"Now can they do everything that way? No they can't. But the days of gridlock will come to an end if Republicans get 50 votes in the Senate."

Among those changes could include a repeal of Obama's health-care legislation.

"You can repeal Obamacare though budget reconciliation, which does not require 60 votes and it's not subject to amendments," Duffy says. "It’s an up or down vote. It's one of the easiest ways, it can't be filibustered."

Whatever the outcome Tuesday, Duffy suggests that the next few months will set the tone for the next few years anyway. "How willing they are to co-operate with each other. We're going to learn pretty early."