WASHINGTON — The fight for the Senate has shifted significantly over the past weeks, with fierce races breaking out in states where they were not expected and other contests dimming that were once ablaze with competition.

With less than two months until Election Day, the Senate landscape is both broader and more fluid than it has been in years, with control of the upper chamber now anyone’s guess. Both parties have seen new opportunities and new challenges, but the net result is that Democrats appear to be in less danger of losing the Senate, while Republicans have a more difficult path to gaining the majority.

Connecticut may be the biggest surprise. Two years after a decisive loss in her first Senate campaign, the Republican candidate, Linda E. McMahon, a former professional wrestling executive, is surging in polls. Wisconsin is also now tilting Republican. Democrats face blistering advertisements financed by “super PACs” in states they once thought were secured, and the tight presidential contest in swing states like Ohio, Florida and Nevada is keeping Senate races there closer than anticipated for both parties.

Democrats are now strongly competitive in races for the seats in Indiana and North Dakota, where the Republican candidates — who were expected to walk away with those races — have exhibited weakness.