No it's not. It comes and goes. In 2002 and 2004, to my recollection, the Republicans looked unbeatable as well. They had the Presidency and both houses of Congress at that time, people were saying the Democrats were finished if they couldn't put up a better showing against the unpopular Bush (even at that time).
Fast forward 4-6 years and people were saying "The Republican party is finished. Obama is issuing a new era of Democratic rule like FDR, and demographic changes mean Republicans will never win sustained major elections again under their current platforms." The Democrats had such control that they were able to ramrod home National Healthcare, which is wildly unpopular in the US in general. Whoops, then 2010 came and Republicans took back the House and Senate, and then the only reason they didn't win the Presidency in 2012 was really because of Obama's cult of personality.
It will swing back.