New Delhi, Oct. 17 -- Republicans' nearly $1 billion war chest was expected to help the party overcome a difficult political environment ahead of the US midterm elections. But despite a massive spending blitz, polling trends have barely shifted, forcing the party to divert funds from potential victories to defending vulnerable seats.

Republican groups spent more than $740 million on congressional races between Labor Day and October 14, including at least $220 million from groups linked to President Donald Trump, according to a CNBC analysis of AdImpact data. Billionaire Elon Musk has separately donated more than $276 million to Republican groups since July.

Yet Democrats continue to lead in several competitive Senate races, while the Co...