Many point to a strong sense among Americans that the country is on the wrong track, with 66 percent of Americans believing the country is on the wrong track, according to a Rasmussen poll published last week.

Meanwhile, the screws are tightening on Democrats as the U.S. economy -- Americans' top issue of concern -- is recovering too slowly for many Americans, as millions remain jobless as the country claws its way back from the worst recession in decades.

"The question is not if the Republicans will take the Senate, but by how much," Republican strategist Ford O'Connell told Xinhua.

Indeed, pressure mounted on Democrats Sunday as a newly-released NBC News/Marist poll showed Republican Senate candidates in strong positions in key U.S. states in the country's southern region, which puts control of the Senate within Republicans' reach.

In the state of Kentucky, Republican Senate minority leader Mitch McConnell leads Democrat Alison Grimes by nine points, and in the state of Georgia, Republican David Perdue leads Democrat Michelle Nunn, according to the poll. Losing those key states could spell trouble for the Democratic Party.

Still, a GOP win on Tuesday would be no guarantee for the party to win back the White House in 2016 against the Democratic Party's likely candidate Hillary Clinton, former First Lady and Secretary of State in Obama's first term, experts said.

That is because U.S. voters in midterm Congressional elections tend to be white and conservative, whereas presidential races see many single women, minorities and young people -- typically among Democrats' base -- come out to cast their ballots.