The battle for control of the U.S. Senate has tightened dramatically with just six weeks left before Election Day, according to a fresh wave of polling and market-based forecasts that shows Republicans' grip on the chamber slipping.

Wall Street research firm Piper Sandler raised its odds of a Democratic Senate takeover from 40% to 45% this week, citing shifting polling momentum in a handful of competitive races. It's a notable jump for a chamber Republicans currently control, and one that would have seemed unthinkable to many strategists just a month ago.

Numbers Even Republicans Are Watching Nervously

The most eye-catching data point is out of deep-red Idaho, where a poll from Advanced Targeting Research conducted Sept. 13-16 showed independent challenger Todd Achilles leading Sen. Jim Risch, 50% to 39%. Idaho hasn't sent a Democrat or a Democratic-aligned independent to the Senate in decades, and a double-digit deficit for a sitting Republican incumbent there would have been dismissed as noise in a normal cycle.

South Dakota is nearly as tight: a Public Policy Polling survey from Sept. 8-9 had Sen. Mike Rounds up just three points on independent Brian Bengs, 45% to 42%, with 13% of voters still undecided. Meanwhile, in reliably blue Rhode Island, Sen. Jack Reed holds a commanding 54%-26% lead over Republican Raymond McKay, according to an August University of New Hampshire poll — a reminder that not every race has moved.

MIDTERM MOMENTUM IS SWINGING HARD TOWARD DEMOCRATS

Election forecasters have long noted that the party holding the White House typically loses ground in midterm elections, and analysts at outlets including Nate Silver's Silver Bulletin have flagged that some professional handicappers may be underweighting just how consistently that pattern has held. Some analysts argue the traditional "expert" ratings have been slower to move than the raw polling and prediction-market data.

Republicans currently hold a narrow Senate majority, and losing even a handful of seats would flip control to Democrats. Races in Idaho, South Dakota, North Carolina and several other states once considered safely Republican are now drawing outsized national attention and campaign spending as a result.

With Election Day on Nov. 3, both parties are expected to pour tens of millions more into the most competitive races over the coming weeks. Updated polling averages are being released almost daily as the map continues to shift.