Democrats have a message for voters during the final, frenzied day of campaigning: All is not lost.

Party leaders insist they can still hold onto the Senate -- their last bastion of power on Capitol Hill -- and are spending the remaining hours before the polls open on Tuesday trying to convince their voters not to give up.

"I don't agree with the oddsmakers," Vice President Joe Biden said in an exclusive interview with CNN Chief Political Analyst Gloria Borger. "I predict we're gonna ... keep the Senate."

Everything would have to break for Democrats just right for Biden's optimism to carry the day. New polls in states that Democrats must win if they have any hope of keeping the Senate aren't promising.

In New Hampshire, a WMUR poll released Sunday shows Democratic Sen. Jeanne Shaheen and Republican challenger Scott Brown in a deeply competitive race. The poll puts Shaheen's support at 46% with Brown at 43%. The race narrows further -- with Shaheen at 47% against Brown at 45% -- once undecided voters are asked who they are most likely to support.

The numbers are troubling for Democrats because a WMUR poll just a few days earlier showed Shaheen leading Brown 50% to 42%.

A Quinnipiac University poll in Iowa shows the Senate race there in a dead heat, with Democrat Bruce Braley and Republican Joni Ernst at 47%. That could provide some comfort to Democrats after a Des Moines Register poll had Ernst up by 7 points over the weekend.

Though Republicans appear on track to take the Senate majority for the first time in nearly a decade, polls can be wrong and might miss important shifts in the electorate during the final hours of the election.

But veteran Republican power broker Haley Barbour told CNN's "The Lead with Jake Tapper," that it was looking good for his side.

"I think it is more likely than not Republicans will have a majority in the Senate after these elections, but it's not a certainty, never is in politics," Barbour said.

"People shouldn't take things for granted."

Democratic National Committee chair Debbie Wasserman Schultz however said her party's superior get-out-the-vote operation would be the difference maker.

"We are going to hold the Senate tomorrow night. We have, going into election day, a superior ground game that has run circles around the Republicans," she told Tapper.

Emotions are running high among exhausted candidates, who are throwing everything into their last hours of campaigning.

Kentucky Senate candidate Alison Lundergan Grimes, a Democrat, vowed not to give up, despite polls showing her well behind Mitch McConnell, the man in line to become the next Senate majority leader.

"You are the messengers that Mitch McConnell can't buy," Grimes said in a campaign rally Sunday, her voice cracking with emotion. "And I tell you, this strong independent Kentucky woman, I've got kick still in me."

CNN's Dana Bash reported at the weekend that Republican strategists have already told McConnell that things are looking so good that he will indeed be the next majority leader.

Democrats might not be giving up but they know the wind is in their face.

Republicans need a net gain of six seats to pick up the Senate in an election in which they are also likely to pad their majority in the House of Representatives.

The GOP is expected to cruise to victory in Democratic seats in South Dakota, Montana and West Virginia, which would leave them just three seats short of a majority.

And latest polls show the GOP leading in Democratic-held seats in Arkansas, Alaska and Colorado.

Democrats, however, hold the advantage in North Carolina.

To have any hope of halting the Republican tide, Democrats would have to come from behind in several seats on the GOP hit list and push races in Louisiana and Georgia into run-offs. The party is insisting that its vaunted get-out-the-vote operation will make things much closer on Tuesday than they seem now.

Republicans are making the election a referendum on the increasingly unpopular President Barack Obama. The GOP would use Senate control to make him a lame duck during his last two years in power.

Buy if they do win the Senate, Republicans will fall short of the 60 seat supermajority that they would need to thwart Democratic delaying tactics.

And they also know they have a tough map of Senate seats to defend in 2016 — a presidential election year in which Democratic turnout is likely to be higher. So they may have an incentive to show they can govern or to compromise with Obama in some areas like tax reform or trade deals.

The votes have not yet been counted in this election, but the next White House race effectively begins on Wednesday morning.

Likely candidates on both sides are hitting the Senate battlegrounds — and boosting their own prospects in 2016.

New Jersey's Republican Gov. Chris Christie is wrapping up his campaign marathon that has seen him hit 10 states since Friday. He is in Rhode Island, Michigan, Maine and New Hampshire on Monday.

Another Republican with 2016 in his sights -- Sen. Rand Paul -- is making seven stops with McConnell in Kentucky.

Mitt Romney, who two years ago lost the presidency to Obama, travels to Alaska to give GOP Senate candidate Dan Sullivan a last minute boost.

And Bill Clinton will cap his own long campaign swing with an event with Democratic gubernatorial candidate Charlie Crist in Florida — a swing state that will see a lot of his wife Hillary Clinton, in her presumed presidential run.

(CNN)