GOP-controlled Senate? Republicans hold the advantage in mid-term races
President Barack Obama speaks at the Pentagon in Washington, DC on Sept. 11, 2014 (AFP)

President Barack Obama's Democrats rejoiced when early September polls gave them a fighting chance to hold the US Senate in November's mid-term elections, but new data Monday suggests they are slipping.

With just five weeks before Americans decide who will represent them in Congress, election modelers at The New York Times,, The Washington Post and Huffington Post show Republicans making important gains in their bid to retake the upper chamber of Congress.

Republicans need a net gain of six seats to win back the Senate. The Times forecast boosted those chances from 55 percent last week to 67 percent Monday, while the Washington Post Election Lab's prediction went from 65 percent to 76 percent Republican.

And the model operated by statistician guru Nate Silver, the most respected election prognosticator in the business, bumped its Republican takeover odds from 55 percent to 60 percent.

"This election is ours to win," the Republican National Committee's chief operating officer Sara Armstrong wrote in a weekend fundraising email, one in a mounting pile of pitches by both parties to their supporters.

With Democrat-held seats in Montana, South Dakota and West Virginia essentially being conceded to Republicans this cycle, the GOP needs to grab just three more seats -- and hold their own -- to win control of the 100-member Senate.

That puts the focus on a handful of tossup states like Alaska, Arkansas and Colorado, all of which feature embattled Democrats who polls show have recently lost ground to their Republican challengers.

And in the breadbasket state of Iowa, Democrats risk losing a seat that retiring Senator Tom Harkin has held for 30 years. The race was seen as a dead heat recently, but a weekend poll by the Des Moines Register showed conservative Joni Ernst leading Democratic congressman Bruce Braley by six percentage points.

Democrats are finding themselves in a tough political spot; Obama is unpopular, especially in swing states, meaning candidates there are not expected to hit the campaign trail with him.

Complicating a Republican path to majority, Democrats are eyeing a possible flip in Republican stronghold Kansas, where veteran Senator Pat Roberts faces an unexpectedly strong independent candidate.

Republicans already control the House of Representatives and are expected to keep it. Lawmakers elected in November take their seats in January and serve through the final two years of Obama's term.